I'm excited about this purchase. (Also given that this was probably purchased with off-shore cash they can't bring back to the states without being taxed, it's pretty much a $0 deal for Microsoft)
I'd like to see Microsoft bring the power of Azure to bear and let us have much larger worlds, with support for more players in the world at a time.
Personally I'd like to see a Smartglass app as a companion, as an easier way to access my inventory, see the map of the world, and maybe even remind me of which things I need to put together to build something else.
I hope they continue to build cross-platform and allow the mods to keep on being developed. Be nice to see some official developer tools customised for making mods too.
Looking forward to seeing where Microsoft take this.