Crossovers have always appealed to me even if I've seldom had the change to experience them (in large part because of the kind of gameworld specific rules GURPS goes out of its way to avoid) so one all encompassing rule set is very attractive. So many things even in a brief skim of AD&D 3.0 reminded me of GURPS that, to borrow your analogy, comparing the two is like comparing Win95 to '80s era UNIX: Sure, Windows does a lot better, because coming along a decade later allowed it to provide all kinds of functionality that simply wasn't possible in the '80s, not to mention benefiting from things the other system innovated.

Forgive me for saying so, but it seems like the simplest way to avoid that problem is to do away with classes altogether and just have people give their characters whatever "class" they want by building the appropriate skill set from the same pool of character points all characters get. You don't have to playtest it much for balance because it's inherently balanced by the point system. Having classes AND character points seems like the worst of both worlds (part of what was wrong with 2nd ed. proficiency slots being a function of class as well as intelligence; everyone but fighters got screwed in WPs, and they got screwed in NWPs). Not only that, but GMs can create their own class templates in lieu of or as an alternative to published ones.

But that's precisely the point: Because the playtesters have already researched and balanced all that stuff for you you don't HAVE to spend time on it to incorporate it into your games. You don't have to incorporate most of it, in fact, but if you choose to do so you can, and it doesn't become a hideous time sink. That liberates the GM to concentrate on creation instead of mechanics without sacrificing any realism or detail. If you need or want a rule to cover a given scenario it probably already exists in a more realistic and balanced form than you could produce without a lot of work. Your analogy applies again: Once you know any system all the mechanics fade into the background when not in active use; the difference is that GURPS has a lot MORE of that detail if you DO use it, and you're not obligated to use it even if the need arises. Again, that's why there's Advanced Combat for people who want to diagram every position, orientation and action, plus Basic Combat for people who just want to roll a few dice and move on again (plus an optional "flesh wound" rule for people who want the kind of combat intensive campaigns that spell quick death with the average 10 point HT in the standard rules).

Flavor, yes; rules, not necessarily. I can't speak to how well 4th ed. GURPS does it; to be honest, from what I've seen of Character Books, Gamemaster Guides and the like it seems like they've gone more in the direction of TSRs "let's see how many accessories we can sell" model. Comparing GURPS 3rd ed. to its contemporaries usually left the latter rather wanting, IMHO. That's before taking into account crossovers, which most other systems can't even begin to handle without a LOT of GM work that shouldn't be necessary.