This is a much simpler system. I like it. It adds some versatility to a class, which is definitely a good thing. The only thing that I see that could be tricky (and I realize that this may be covered somewhere else) is the inherent problem of a character changing over time.
Mike's a Wizard. After adventuring for 5 years, he realizes that he's satisfied with his body of knowledge and would like to move on to learning something else. He wants to learn the ways of the Rogue, too.
In 3.5, Mike would have 8 levels of Wizard and then start taking Rogue. In 4th, though, he would probably just start taking the multiclass feats as he levels up. He may even then take a Paragon Path for a Rogue to emphasize his growing skills. What's the problem that I see? Well, he's just as powerful as he's always been, even though he's not focusing on his spellcasting.
That's the only problem that I have. Thankfully, it's a minor one that I can easily live with. I'll probably stay with one class, throw in a little bit of something else, and then happily take my Paragon Path. I'm not too worried about a character making a major change like that.
In retrospect, though, after taking enough of the multiclass feats, it would probably make sense to retrain to being a Rogue with Wizard feats. That would show that his arcane abilities are starting to atrophy and he's becoming specialized as a Rogue.
Well, I think I just completely solved my minor "problem" with the whole multiclassing thing.
For those who have problems with the whole "only 2" thing:
Picture, if you will, a college kid. He's studying Law, Medicine and Education. He does his internship to be a doctor. After school, he becomes a teacher who practices a little law on the weekend. Eventually, he decides to just focus on teaching.
You can only really have one career. Anything else is a job. As time goes on, our hypothetical student probably forgot a lot of his medical stuff. When he's old and ready to retire, he probably won't be up to date on current laws and will probably have forgotten most of the things he used to know.
In 3.5, they would have you believe that, at the age of 75, this guy would still be as fine a doctor as he was 50 years before, he could be thrown into a courtroom after being out of one for 45 years and be just as effective as he was back then, and he was as good a teacher as he was a few years before that (which makes sense). Technically, though, 3.5 wouldn't be able to recognize the teaching and lawyering as happening at the same time.
In 4E, though, by the age of 75, he's forgotten nearly everything about medicine, enough that it doesn't even really matter anymore. He spent most of his life as a teacher, so has all of his class levels there, and he might have a few "feats" when it comes to being a lawyer.
Is 4E perfect? Nope. I'm sure that you can come up with arguments against it. Does it make more sense than 3.5? Yes.