That has more to do with obsession cycles than any intrinsic feature of the game, but my gut feeling answer is that it's much more limited than 3rd edition and its variants.
This is mostly nonsense. 4th Edition isn't significantly less complicated than 3rd Edition, it just increases tactical complexity at the expense of character complexity. With the exception of fighters, rogues, and other non-magic-using characters, 4E characters have fewer moving parts than their 3.x counterparts, and there's much less variety in the kinds of parts they use. However, this reduced complexity means its easier for the designers to ensure that all those moving parts have interesting interactions with each other, and that the characters have interesting interactions with the rest of the party.
Which means, for pure combat, and even combat with motivational doo-hickeys (I'm fighting for the queen!) backed up by interaction scenes, 4E is superior. 3.x characters have an unfortunate tendency to find one best strategy and use it in every fight, to suck up table time with calculation, and to blow through opposition, though that last is mostly because the CR system rests on some assumptions that aren't supported by the game as she is actually played. On the DM side, 4E has much better support for making fights interesting, from making monsters easier to tweak and run to gearing its environmental design advice towards making terrain that gives players interesting choices.
On the other hand . . . 4E doesn't do a whole lot beyond that. There are precious few non-combat abilities, and it's impossible to build a character that doesn't focus on combat. It does have the skill challenge system, which I like a great deal and could see using to use to run a game by itself -- because it's a seperate system that's been bolted on to the main, power-based core where most of the game's complexity resides.
If you want to run a game that revolves around exploration, or bullshit hi-jinks, or anything else where the point of combat isn't to have fun with the fight itself but to cause problems for the players, then whenever combat does come up it will invariably pull attention towards itself and away from the main point of the game. And if you don't use combat much, you're looking at a character sheet that you never use and making lots of character decisions that are never meaningful. 3.x often has the same problem, especially when you start adding in splat-books, but that's a function of the particular moving parts in the system -- what spells your wizard picks or whatever -- rather than being cooked into the arrangement of the moving parts themselves. It doesn't intrinsically assume, no matter how you build a character, that the character will be about "combat and occasionally some other stuff."
When I wrote this post, it ended up being crazy long, so tomorrow I'll pontificate about another, related difference between 3.x and 4E. As a player of mine used to say, "All the characters are the same!"