Continuing that thinking out loud trend ... This is sort of a reply to the above, at least in spirit, but I'm mainly just springing off the sense of both of your ideas rather than the details.
I think Abdul's three sources could work well. What I don't like about the implication of that approach is that characters fit into one source or another. That's fine with "Fighting-Man" and "Wizard", because they do. But Paladin, at least the D&D concept of Paladin, really doesn't. And for that matter, neither does the Cleric.
I guess what I'm objecting to is the seemingly inevitable drive towards a pyramid structure of classifications that seems to hit D&D designs so hard--even when you've got things that obviously don't fit that structure. We get this whether we start with classes roles or sources or whatever. It's almost like biological classifications.
I also think this is why in any given design, there are elements that become rather vestiges. Power sources are the most recent example. They were supposed to be meaningful in the design space at one time. But because they had to fit into some kind of tight classification scheme, the things that were supposed to be attached to them got gradually moved onto classes, roles, feats, etc. (Maybe. Or maybe that same movement happened for some other reason, and they hung around as vestiges for flavor only. I don't really know, of course.)
Therefore, I think the answer to the problem of ending up with a few giant lists or a huge number of short, stubby lists--is to attach something meaningful but reasonably tight to each element in the categories, but don't make the categories completely hierarchal. Then develop the classes in terms of those elements. Or to answer the question from KS--I think the powers should not be an intrinsic part of the class--but there should definitely be effective, structural limits.
Suppose the following categories, and
all of them have appropriate, meaningful powers and features attached to them. (This is somewhat mutually exclusive with my earlier posts).
Sources: Martial, Arcane, Spiritual
Roles: Defender, Striker, Leader, Controller
Seven lists. But where do you put a "martial defender" power, in 4E terms? Well, is it more about the martial aspect, or more about the defender aspect? Pick one. If you can't decide, tweak it a bit so that you can.
A "fighter" then gets to pick from Martial and Defender mainly, with a modest selection from Striker as a secondary gig. Somewhat like the 4E druid, there are some hard limits on how extreme the ratio can get, but certainly opportunity to skew one way or the other.
Then on top of this, the fighter gets a few things that make them uniquely the fighter. This might just be weapon proficiencies and certain skill/feat selections. Or there might be a small handful of features/powers that only the fighter gets.
Finally, and getting away from the overly simple example, each class should probably also put some restrictions on what can be picked. I'd do this via keywords on the powers/features, which the class had to possess, but you could do it any number of ways. Basically, this is a flavor thing, with mild balance implications, and allows some useful shorthand in a few cases. If you want to give some supercharged Bard-only class features, then you might compensate by more heavily restricting their choices in the Arcane list.
Note that if I were in charge, I'd also make Race a category, with features and powers by race, and then Culture a category, perhaps stealing something similar to the RuneQuest model. So you'd have Cultures: Nomad, Primitive, Barbarian, and Civilized. This would completely remove the need for the "Barbarian" class, and not a few racial feats and work-arounds, but I could live with that. Of course, the willingness to go that extra "bridge" is a good reason why I shouldn't be in charge.