Apps should start faster, since they don't have to wait for the VM to identify hotspots and JIT them. This should also lead to reduced memory usage overall.
I'd guess ahead-of-time compilation means they can "afford" slower compilation times (i.e., a slow compile won't disrupt the flow of user in an app). And that would give them room to do more optimization. But that's totally a guess.
It's a new "runtime." I suppose they don't call it a VM because it's an ahead-of-time compiler. Dalvik is pretty efficient, so don't expect miracles, but ART is probably a good trade of space for speed.
