Short of changing fluids and a battery, what more can the average Joe work on a car built in the last 25 years?
Pretty much everything?
Brakes are easy. Not much has changed.
Water pumps, serpentine belts, and timing belts don't need changing nearly as often as they used to, but they're not that bad on many vehicles.
Spark plugs are generally straightforward. So is the distributor, which is still present on many vehicles in your 25 year window.
Air filters are designed to be changed, though the cabin air filter may be a pain.
Fuel pumps tend to fall into "easy" or "drop the tank" categories, but easy ones are an hour project mostly.
Suspension work is harder, but is certainly still doable, if you've got a good breaker bar.
I've done an idle air controller recently for someone - it was sticking and causing drivability issues.
Headlights/brake lights/turn signals are generally easy, though some cars make it tricky.
And if you want to get nuts, you can totally rebuild engines on newer vehicles if you care to.
The nice thing is that all the common maintenance you used to have to do (points, water pump, carb adjustment, etc) is less common - the computer handles a lot of that stuff flawlessly, and tells you when it's unhappy (seriously - a cheap OBDII reader and phone means the computer will, most of the time, tell you exactly what's wrong).
Now, if by "Average Joe," you mean literally an average car owner who drives it and ignores the warning lights and noises until something expensive fails, then takes it to the mechanic and bitches for the next 6 months about how expensive the shop was, yeah, there's not much that can be done to help there. But if you're willing to dive in, a LOT of stuff can be done with a simple set of hand tools and an afternoon.
I'm not going to total up my auto maintenance recently because I've got a truck I'm slowly converting from "takes it to a shop for everything" spec to "enthusiast spec" and that's cost a good bit, but I'm getting a better engine out of the process, and replacing 20 year old rubber is always good for reliability. I've got another $1500 or so of parts I'll order once I have more space to work, since redoing the intake/exhaust system to resolve boost leaks and pre-turbo exhaust leaks is a major project (and I may as well swap the turbo wheel out with an upgraded one while I'm in there, since... well, I'll have it off - may rebuild the turbo too if it needs it). On the other hand, I fully intend to keep this truck for the rest of my life, and it spends it's life moving large things and towing heavy things.