An app store would just be an apple-blessed place to get AppleTV specific applications. There are already lots of them, but you have to hack your AppleTV to use them.
All they need to add is an app store, then let third parties take care of the rest. This is basically what we've been doing with the current appleTV hacks and Boxee, keeping the core functionality of iTunes/iTunes store and then adding the extras we need, like perian, playing off network shares, couch surfer, boxee, emulators etc. The simple addition of an apple blessed app store would give us at least some of this (while we'd miss out on game emulators, experience on the iphone shows us that we would more than make up for that omission with new games actually meant for the device).
First off, I love Boxee! I'm building a MAME Cocktail cabinet and media jukebox and I'd love to use boxee as the front end. It would be nice to have a MAME or other game emulator launcher.