That sort of thing has always been my biggest concern with games like this. Blizz seems to be doing the right thing here, especially since they've stated that the camera won't be cumbersome. I think it's going to make campaign authoring even easier. It's also going to challenge people to create better storylines and characters and missions, rather than focusing on cheap editing tricks and the like. The tools will all be there.
Total conversions are likewise easier to pull off.
It's probably a toss-up between the single player campaigns and the actual fun of multiplayer with all these new races. Mostly though, I think it's the idea of new races, and six total. The Burning Legion sounds terrific. It's pretty much an open field at this point. I'd like to get an idea for just how much of the gameplay has been altered, beyond the idea to recruit additional troops and use heroes as walking farms, in a sense.