I think Bauglir mostly hit upon the things that were important to me. Put relatively simply, the ideal class-based system for me (caveats below), would be one that has a lot in common with 3.5E but where many of the base classes were actually good at their job.
What I mean by that is a game where you can find classes that realize common and cool archetypes "off the rack." Looking for a lightly armored guy inspired by the Grey Mouser? Look at Ranger and Rogue, those will work fine. Want a heavily armored tank who can take punishment? Check out these Barbarian, Knight, and Paladin variants. I'd like the same thing for spellcasters, too, but D&D I find it easier to describe warrior concepts this way.
And, all of that would coexist with something along the lines of 3E's mix and match multiclassing system. So, that someone who is particularly interested in mucking around in the system could achieve those same ends in very different ways with different mechanics, and so on. To a non-trivial (but highly imperfect) extent, 3E managed this by the end of its life. A sneaky, agile melee fighter type can be pretty well-realized with a Swordsage (out of the box), an Assassin (again, pretty much out of the box), or more exotic combinations like a Melee-ficer.
Caveats: I am a particular kind of player. I'm going to spend a lot of time working on my character concept and my character build, and I'm comfortable and even prefer talking about a campaign at some length before it starts. So, I'm sort of the opposite of the strangers get together to play a game comfortably without any fuss model. If D&D wants to go that way, then they'll leave me behind, and that's fine -- I can understand the appeal of that sort of game.
That being said, I think it's probably an extremely poor strategy on the designers' part. D&D has for a long time been just too fiddly, tactical, and mechanics-heavy of a game to be suited to that. And, I'm not talking about comparing the OD&D style of games to 3E and 4E. There are a lot of nice games out there that aren't stripped down or even "rules-lite" but that are a whole hell of a lot less fiddly than the previous 3 editions of D&D. And, that fiddliness has probably been its subtle attraction to a lot of people.