Hussar
Legend
I don't think there's very much innovative in the mechanics of 4e, unless you consider "New to D&D" the same as innovative. In many ways, the mechanical changes seemed like an attempt to get back to results similar to those of earlier D&D, where 2e and (much more so) 3e had moved towards the "Magic is Better" paradigm. Now, some of the setting elements they use do seem pretty innovative. As for DDi, if the apparent original intention had been carried through, then I think it would have set a new standard in terms of integration of computer tools in play. That would in my opinion have been a very good innovation, though since it fell apart in the implementation I'm less inclined to be generous to it.
Now this I will disagree with. One of the biggest innovations in 4e is breaking the initiative paradigm. That's not only new to D&D, but, new to a lot of RPG's. Yes, I know you had AOO's in 3e. Sure. But, that was reactive, not pro-active. You, the player, could not take an action outside of your turn in initiative.
4e radically changed that. Now, every class can take actions outside of their turn in initiative, and, not only that, but can grant other people actions outside of their initiative as well.
It's a very big shift in play. Players have to pay attention all the time, because, at any point in time, you can possibly be called on to act. Previously, you could largely walk away from the table when it wasn't your turn, and, so long as you came back before the DM's turn, you would not miss anything.
3e's big innovation was tying everything to the PC generation rules. Everything in 3e is made just like a PC. For D&D, at least, and a lot of RPG's as well, this was pretty new. The idea of creating a balanced framework for making virtually anything you wanted in the game, either as a PC or an NPC, was pretty unique to 3e. Gave DM's all sorts of tools that, previously, were largely only seen in point buy games.