Well, we already know that apps can register to handle certain voice commands, which puts it in front of Siri and Google Voice Search as far as I'm concerned. It also allows an application to talk to you in the background (see the demo video of the INRIX app alerting the user about traffic as soon as they got in their car) which is pretty awesome.
It'd be nice to have the natural language support, but I'd rather see the functionality first and it looks like Microsoft is covering that well. I'm sure some of the built in apps (Calendar, Music, etc) will have support for it as well.