JJ_Jeep, this is very helpful, being more specific about actual work. In looking over your list, it also occurred to me that there must be proper instruction provided for each of these tasks, otherwise the difficulty rating is irrelevant. The outcome requires that each job will be performed properly, and I'll illustrate what I mean, using your graduated difficulty tasks as an example. I'll touch on the tool requirements and how I would rank the task if the instructions and how-to steps were thorough and the right tools available. Any rating will be relative to the quality and scope of the instructions.
Using your list and the 1998 Jeep TJ Wrangler as an example:
Air filter change—This can be a simple job, a #1 in difficulty, if you don't get debris into the vulnerable engine side of the filter, you seat the filter properly, and the filter seals when you snap the air box top into place. Tools: Clean, debris-free hands.
Rotate tires—Also a #1 difficulty level job if you use chocks on flat ground and the vehicle does not roll away, the jack is secure and you use safety stands placed properly, and the vehicle does not fall to the ground or cause physical injury, you don't damage a brake caliper trying to put a front wheel/tire into place, you center up each wheel before installing the wheel nuts, you don't strip a nut and stud, you bring the wheel nuts up in cross and even steps, you tighten the nuts evenly and securely to the required torque listed in your owner's manual or, if you have a four-way hand wrench or the generally pathetic OEM nut wrench and know the significance of torque and can make a safe judgment call on properly and adequately tightening the nuts, you can verify later with your torque wrench, and you don't over-tighten a nut and either snap off a wheel stud or scarf out the nut seat in the wheel rim. Use proper sockets, extension, breaker bar, ratchet or 4-way, and verify torque with an accurate torque wrench.
Replace headlight—Also a #1 difficulty level if you can remove the headlamp trim ring properly, know which screws to loosen when removing the lamp hold down ring (not accidentally turning the lamp angle adjuster screws and rendering the headlamp out of adjustment when you finish), you unplug and plug the old and new headlamp properly and make sure the wires go back through the lamp bucket without a sharp metal edge pressing against the wire insulation, you place the lamp hold down ring squarely on the new lamp and make sure the lamp seats into its locating notch properly, and you secure hardware, tightly enough to stay secure yet not too tight where you'll strip the screw threads. Proper tools include various screw drivers or Torx bits on a Jeep.
Battery Change—Also could be a #1 difficulty level if you remove the negative terminal first, then the positive terminal, you note the routing of wires as you detach them, you unclamp the old battery and make sure it's loose before lifting it out, you wear protective gloves when handling the battery and avoid carrying the battery against your new denim jeans when you walk away from the Jeep, you place the new battery in the case squarely and clamp it securely, you attach the positive terminal first and secure the nut without stripping it or breaking the soft terminal end, you attach the negative terminal last and make sure you secure it properly so that both terminal ends are tightly clamped to the posts. Use a box ended wrench for the terminal nuts, ratchet/extension/socket to reach the battery clamp bolt if a side clamp or to loosen the nuts atop the J-bolts if there is a battery top hold down frame.
Oil/filter change—This could be a #1 difficulty if you don't strip the drain plug during installation and make sure it's secured to the proper torque so it doesn't loosen and fall out on the interstate or a remote trail; the filter can be a messy job, you must make sure the old filter gasket comes off with the filter and the new filter has a clean gasket with a thin film of fresh oil on the gasket, the filter stand must be clean, no risk of debris finding its way into the exposed opening with the filter removed, you must keep the filter clean where it faces the block; priming the filter is always a good idea, as 95% of bearing wear is at startup and a dry oil filter adds to the problem, priming must be done with clean oil in the filter and just enough oil so that when you bring the filter to the engine mounting stand on an angle, the oil does not run out of the filter, the filter must be secure against the block to prevent leaks and loosening, some high performance filters like a K&N actually require a specific torque setting to prevent the filter from falling off in service. Tools include a socket and ratchet for the drain plug and a sensible filter wrench that will not collapse an overly tight oil filter canister and leave you struggling to remove the oil filter with a drift punch and hammer.
Flush/Fill coolant—A difficulty level #1 if you loosen the drain cock and install it carefully, you fill the system with the right mix of anti-freeze coolant (not too much and not too little anti-freeze protection) to the top of the radiator neck, then fill the reservoir to at least the COLD line, then run the engine with the cap on the radiator securely to remove air blocks and circulate coolant, including opening the heater flow, allowing the engine to reach operating temperature then running a few minutes more with the thermostat open, then letting the engine cool down completely with the cap still installed to siphon coolant from the radiator overflow reservoir, then top the reservoir to the COLD line again with coolant and do this check once more. Tools required are a pliers and ratchet/sockets/extensions and a torque wrench if you also change the thermostat, making sure the new thermostat seats properly, in the correct direction, bolts have sealant on bolt threads tightened to proper torque, gasket is new and seated properly.
Replace serpentine belt—Could be a difficulty #1 level if the belt gets routed properly and fits the grooves to prevent chewing up a new belt, you tension the belt properly to prevent slippage (loose) or knocking out your alternator, water pump or power steering pump bearings (over-tightened belt), you secure all hardware to proper torque specifications. Tools are a ratchet, sockets, pry bar, and good sense not to pry against fragile parts or the power steering reservoir housing when tensioning the belt.
Tune up (new plugs, wires, cap, rotor)—Could be a difficulty #2 level if you remove and install the spark plugs without stripping a spark plug thread or, worse yet, the cylinder head threads, you use a torque wrench to secure the plugs to proper torque, you gap or at least confirm the plug gaps before installing them, you do not rip a plug wire (intended for reuse) out of its insulator by pulling on the wire instead of the insulator with an insulator pliers, you snap the new plug wires onto each plug securely, you remove and install the cap and rotor making sure the rotor seats in its notch on the distributor shaft and the cap seats squarely in its notch before tightening the cap screws, you remove and install wires individually to make sure each wire routes to the correct spark plug, and you secure wires in the clips just as Jeep intended. Tools are a ratchet, extension, screw drivers, plug socket (insulated type), a 3/8" drive torque wrench, spark plug insulator pliers if you're reusing the wires, plug gap tool to check or adjust plug gaps.
Replace alternator—This might be a difficulty level #2 if you can reach the hardware with your sockets, ratchet and extension, you handle the wire insulators with care, you do not over-tighten small hardware and strip terminal threads, you align and adjust the belt properly, and you re-tension the belt to specification. Don't forget to disconnect the battery negative cable while all this is going on...You need a torque wrench, pry bar and torque specifications for hardware.
Replace power steering pump—Same precautions as the alternator and this could be a difficulty level #2 if you don't pry against the fluid reservoir, make sure you use proper flare nut wrenches on hose fittings to prevent rounding nut corners, you secure fittings squarely and properly, you fill the reservoir with clean power steering fluid and prevent any debris from entering the system when fittings, hoses or other parts are open to the atmosphere. Debris in the steering gear will ruin delicate parts. Re-check fluid after operating the gear in both directions.
Change transfer case fluid (special fluid transfer tool helpful)—Definitely a difficulty level #1 as long as you don't strip the aluminum threads on the transfer case when installing the plugs, you secure the plugs to proper torque specifications, you use the correct tools for the drain and fill plugs and you do not get debris inside the transfer case. Use correct fluid here. If factory recommended, use the correct sealant on plugs and do not allow excess sealant to slough into the transfer case, it will clog the pump pickup screen.
Change Manual Trans fluid (special fluid transfer tool helpful)—similar to and essentially the same precautions as the transfer case, use the correct tools, and as JJ mentions, a fluid pump to prevent spilling the expensive new gear lube everywhere. If plugs are secure and the fluid level is correct to the factory fill point, this is a difficulty level #1—a plug falling out of the transmission case or leaking, or a low fluid level on the refill, could result in expensive parts damage, dropping this to a level #4-10 project.
Change Diff Fluid (special fluid transfer tool helpful)—Same as manual transmission, here you have a diff cover gasket, seal or RTV sealant to address. Avoid excessive use of RTV, make the sealant bead as described on the product or in your Jeep shop manual, keep all areas and parts clean, and fill the axle/diff properly (allowing for that lift kit rear pinion tilt and all of that stuff). This could be a difficulty level #1-2, however, if the diff leaks or the fluid level is off, if you strip the diff cover bolts or over-tighten them and warp the diff cover, this deteriorates to a level #4-#10.
Install car stereo—This could start out as a level #2-3 difficulty unless you misroute wires, pinch wires, short out wires, forget to disconnect the battery negative cable before starting the job, break the plastic dash trim, make poor wiring connections that loosen and short later, use solder connections but don't know how to solder, and the wires loosen and short to ground, you forget to fuse the circuits and a subsequent short burns out yards of wiring. In such cases, difficulty level increases to #4-10. A simple, direct retrofit stereo could be considered a #2 difficulty level with care applied to removing dash trim and routing wires safely.
Replace a fuel injector—JJ_Jeep did this by the book and had little difficulty, and a level #2-3 difficulty would be possible. If you break insulators or wire plug connectors, damage other injectors, nick an O-ring or fail to secure and seal all of the injectors, this would be more difficult. If you need to detach any clip-on fuel hoses, use the correct hose release tools, or this job will rapidly deteriorate to a #4-10 level.
Replace steering gear (Pitman puller and 16" crescent wrench*)—The pitman arm replacement is enough of an issue for me to do a specific topic or how-to article on it. Installing a pitman requires the puller JJ_Jeep describes plus knowledge of how not to damage a steering gear. This could be a difficulty level #2-3 if you hold the pitman arm in a heavy vise while installing the gear off the vehicle. Gear on or off the vehicle, the pitman arm should never be forced against its extreme left or right turning position when tightening or loosening the pitman nut. This will damage the internal gear parts...I set the pitman arm and gear on center for this chore and use either the vise approach (gear off the vehicle) or, when the gear is on the vehicle, with wheels/tires on the ground and pointed straight ahead to prevent the pitman arm from moving toward its extremes. Never lock the steering column to hold the gear on center during this procedure, or you can damage the steering column locking mechanism when you torque the pitman nut! If you follow these precautions and use a heavy duty torque wrench with large socket, and keep the pitman arm close to center position while tightening the nut, you'll keep this task to a level #2-3. Neglect these steps, and the resulting damage will raise the bar to a #4-10.
*Caution to JJ_Jeep: I would recheck that pitman nut torque with the gear on center, wheels on the ground and weighted, and the steering column unlocked. It's unclear whether a 16" Crescent wrench can achieve the required 185 ft. lbs. torque (1998 Jeep TJ Wrangler factory Mopar service manual specification). I use an air gun that will hit an accurately predictable 160-180 ft. lbs. torque to seat the arm and nut, then I complete and verify torque with my 0-250 ft. lbs. torque wrench. Please do a re-check, JJ.
My point here is simple. I could go through the pending projects in similar fashion and throw ratings at each. The ratings are only meaningful if the instructions are detailed enough to eliminate the safety hazards, risks and pitfalls, those factors that would be unknowns to an inexperienced mechanic or anyone doing a particular project for the first time. From this exercise, one thing is clear to me: If we use a rating system, we also need to be thorough in describing all tasks involved and any known pitfalls, challenges, safety cues or projected issues.
Also worth mentioning is that we each have expertise in one area, maybe lesser skills or interest elsewhere. Whether the grading is a numbering system or images like wrenches (a good idea, visually), this should also imply that the skill rating is by someone who has already performed the specific task, on this type vehicle or equipment, or at least has a professional sense for the project at hand and access to factory service manuals for details and specifications.
We're getting somewhere...This is a good discussion, and thanks, JJ_Jeep, for moving the process forward!