I'm afraid I agree with Lancefighter here.
Spell mechanics in DotA give casters (Intelligence heroes generally) a much more defined role than AP heroes have in League. Specifically, they dominate the early game, are still powerful in the mid-game, and taper off in the late game.
In fact, in DotA a full team of 5 casters could win the game if you finish it early enough. By pushing with your powerful early-game advantage, you can destroy your opponent before they ever gain enough strength to resist you.
And before you go contesting whether or not it's possible to win a DotA game without carries, take a look at these statistics:
http://tinyurl.com/8trarksThe most successful number of carries on a team wasn’t one. It wasn’t two, or even five. It was zero. A team with no carries on it at all tended to win 14.1% more than average.
That really defeats the notion coming from inexperienced players that carries in DotA 2 are overpowered I think.
This is the kind of emergent gameplay I was talking about that DotA has and that League of Legends doesn't. By giving casters a very definitive role of early-game dominance and late-game support, it makes the gameplay much more unique and shifting. It also makes the early-game a lot more exciting, where LoL early-game is often (but not always) a lot of farming with a couple ganks every now and then, and then late-game is all about team fights.
It's just personal, of course, but I much prefer each hero having an extremely defined role. I don't like how AP muddles all the roles together. The only real difference between AD and AP in my experience is that AD usually does it's damage consistently, where AP does it in short bursts. That's a slight difference, but certainly nowhere near as meaningful as the difference between casters and carries in DotA.