What can I control with voice control

Pretty much the nav/entertainment/phone stack, so no climate control. If you go to Chapter 4 of the Navigation/Audio manual (the 2nd biggest of the manual stack!), there's a pretty complete discussion. It's a bit more capable than the manual explicitly describes, but if you don't make/take phone calls in the car, it's mainly for running your sound system and maybe navigation (though that can take more patience).

The single biggest use (for me, anyway) is satellite radio, where it seems to be the only practical way of navigating the XM channel list without memorizing the number of every channel you listen to. So you can say "Play satellite radio," then "Play station Classic Rewind." Once it's on, you can change stations with just "Play station Bluesville." Folks process information differently, but I find it easier to remember station names than numbers, and the radio's preset "buttons" don't show station names, making them largely useless for XM.

 

Gen2 would let you adjust the climate control set temperature by saying commands like "Seventy-four degrees" or "I'm hot" (or cold). I suppose they removed it when they gave us physical climate controls in Gen3, but I found the voice commands more convenient to jump directly to a specific temp setting.