Basically, the framework of abilities for the various classes are the same. All classes generally get the same number of slots for power and feats. I'm not exactly sure why there is such a big issue about the fact that the steps you go about in creating or levelling up a wizard is very similar to that of a fighter.
One downside comes to mind: the threshold for perceived imbalance is much lower.
In 3E, a 7th-level wizard is casting 1 or 2 4th-level spells. A 7th-level sorcerer is casting 5 3rd-level spells. The wizard needs to pick in advance, the sorcerer can choose spontaneously as needed. But the wizard can expand his repertoire if needed, whilte the sorcerer is locked into his until next level. Which one is better?
I'd say the wizard, but that's almost 10 years of 3E experience speaking. It's also taste speaking; I like versatility. But many people will find enough subjective pleasure in the sorcerer's capacity that they'll prefer the sorcerer, even if there is a consensus that the wizard is more powerful in some objective sense.
Now look at the 7th-level fighter; his top trick might be Whirlwind Attack, whenever he can. The consensus is even stronger that he's weak compared to the wizard, but some people will enjoy the way the fighter works more than the way the wizard works, so much so that they'll prefer the fighter, even if the wizard is certainly more powerful objectively.
In 4E, on the other hand, there's a 1st-level warlock daily which deals 3d6+Cha damage and imposes a penalty. There's also a 1st-level sorcerer daily which deals 6d6+Cha damage and imposes a penalty (I don't have the exact spell names, but the point I trying to make is more general than these two exact powers). They're immediately, obviously comparable, and the warlock daily suffers in that comparison.
Even if you go further afield and look at different roles and power sources, you can still compare powers with an exact 1-to-1 mapping. Paladins get an encounter power at 7th level just like warlocks, and even though different roles and different class features make a lot of difference, 3[W]+Str is still much more readily comparable to 3d8+Cha than Whirlwind Attack at will is to enervation 2/day.
In 4E's unified framework, it's much easier to see imbalances, and taste accounts for less. Whether you enjoy consistency, or a single big bang, or careful preparation, or versatility, the structure of your repertoire of powers and the way the function is always the same, and it's much easier to slip into comparing numbers, and the numbers can only be (to be generous) imperfectly balanced.
To be fair, it should be noted that new classes are changing that in smaller (avenger) or larger (psion, monk) ways.