Well, I've written pretty big walls about this... (Cal, you might remember some vaguely
) Basically, what I've found is that the more classes you have the less varied the play style of each class is. You might have minor alterations in them, but then there's really only one play style, or one strength that each class has. What I would prefer to see is having a smaller number of classes that have different ways to build them, be it through selecting which skills you use (like a guild wars type of thing, or D2 where you either have the skill or you don't), or even a talent system. I realize any of these systems would bear a resemblance to WoW, which a lot of people will instantly see as a bad thing, but I think it's one of many things the game, for all its faults, actually does right.
Now, there's a tendency in WoW for class specializations to fall under the cookie-cutter PvP spec and the PvE spec, which I think is generally a bad thing and why
talents wouldn't be the best way to go imo. Instead, I'd love to see a D2 style specialization where you commit to skills and boost those, and anything you don't put points into you don't even have available. What this kind of system does is allows you to have only 8 classes, but then have a number of viable builds for each class, where you're using different abilities at different times and you have a more unique experience. Each class, however, has the same overall feel, and since you would presumably be able to change your spec (for some fee, of course) you wouldn't have to roll a completely new character to get a slight variation on the general play style you already have. That's my take on the issue.