Why is it so important that building characters of different classes be different? The main reason why you build characters is so that you can play them. The amount of time you spend building a character is likely a fraction of the time you would spend playing it. I figure the important part is that characters of different classes actually play differently.
This isn't necessarily true. I made many 3.5 characters solely for enjoyment, many of which I never played or even intended to play. I would volunteer to make or assist making other PCs for players who didn't care for building or were new. I made all sorts of NPCs for my DMs and even their friends who were DMing completely different games. Point of fact, I got at least as much enjoyment from building characters as from actually playing them.
I'm not alone in this, I know at least one other individual who never played 3e at all, but bought books and made characters as a fun past-time. (Mind you, I'm not claiming anything about how prevalent this behavior is, just that it is one way to enjoy D&D).
I like 4e for a whole host of reasons, although I have my grumblings with some of its parts. I also don't make characters for fun anymore, in large part because they tend to build the same (with respect to my enjoyment at least). Yes, characters do play differently for the most part, but that is only part of what I enjoy.
If I were to change one thing about 4e it would be to make the at-will/encounter/daily framework more general. The number of "power slots" gained and when they are gained can remain fixed, but classes can use them much more freely. I think 4e can support this change, and I wouldn't be surprised if this territory was explored quite fully later in the game's life (the psion is a start!). For example, a class might only gain at-will powers, "encounter" slots might be spent to upgrade the at-will powers in various permanent ways, and "daily" slots might be special and powerful improvements to various at-will powers that only obtain under special circumstances fairly difficult to achieve (a prone and dazed bloodied enemy, for example). Something like that would have fulfilled what feels most natural (to me) for martial characters or a 3.5-like sorcerer/warlock.
It would also make it easier to bring back a "starter character" class that can be really darn effective without the resource management currently needed. Finally, and it's mostly just a pet-peeve, but I really hate the (not quite but almost mandatory) power swapping as level increases. If a character wants to use the sleep spell all the way from level 1 to 30, and have it remain effective the whole time, I think that should be an option for at least some classes. I value character continuity, and the power swapping rubs me the wrong way. Mind you, I think retraining is a good idea and I'm glad it was codified into the game.