Part of what I dislike about 4e is that it's too balanced. Yeah, that sounds loony since poor balance has always been a bugbear for D&D mechanics. But it's true.
Still, there is such as thing as so much balance that there's no difference left. There really should be a tonne of details that make playing a ranger different from playing a warlock, because they're such very different sorts of characters. And yet, they're pretty much the same thing during the majority of a 4e game (i.e., in combat) because their abilities are pretty much the same but with different flavour text.
Magic items suffer this particularly strongly in 4e. A wand in 4e is just a +X to-hit magic weapon with a daily special effect, that can only be used by magic classes. A magic sword, to (not) contrast, is just a +X to-hit magic weapon with a daily special effect, usable only by martial classes. For (real) contrast, magic items in 3e and earlier were so cool that just reading about them was fun.
Jeff Rients puts it well in a blog post:
To me, it looks a little bit like a Euro style boardgame: an exquisitely balanced abstract game about nothing in particular with a whitewash to give it a little context. In this case the whitewash is "D&D but not the boring old D&D you've loved for 30+ years".
Exquisite balance just isn't a selling point in a roleplaying game for me. I know outside of combat (and the non-combat "just like combat" encounters) this isn't a problem with 4e. Considering how much time is spent on the battlemat in 4e, though, it's a huge black mark against the system.