With Mountain Lion and the iCloud feature only allowed on apps from the Mac App Store, this is a lot harder to do than it previously was.

There are a lot of cases where iCloud isn't relevant (at the moment) -- many types of apps are not document-centric and I believe that, going forward, fewer will be.

iCloud should be limited the MAS apps-- I don't want some rogue app being able to access my iCloud data.

Already done a long time ago: http://appbodega.com

(Although I believe Bodega launched a few years prior to the Mac app store)

Sometimes being early is the same as being wrong -- we've seen that time and time again. If a new player were to come along and capture the momentum of this meme, I personally believe that they'd stand a better chance than a player who has been in the space a long time. It's unfortunate, but that's just the way it is -- or always has been.

There is already such an app, Bodega, which was launched before Mac App Store. There was (and probably is) another one from the developers of iusethis.com