Maybe this is obvious but as iCloud rolls out, users are going to add a new criteria to their app buying calculus. “Is it multi-platform?”

Just like the way even us ‘power users’ are getting hooked on Lion’s versioning and auto-saves, even the most die-hard Dropbox supporters are going to find themselves expecting data to migrate between their Macs, iPads, iPhones, and even Windows PCs.

With iCloud, there is no secret incantation, retina scan, or hacking involved. Your data just is. No longer will you have to consider whether the right folder is synced to the right app. Work on one device. Turn it off. Work on another device and pick up where you left off.

There is a price to brain dead syncing. From everything I’ve seen, in order to work, you’ve got to be working on the same app on every platform.

For the first time since the iOS arrived in our lives, using the same app on multiple platforms comes with an added benefit, data bliss. When users look at apps for their Mac or iOS devices, they are going to actively seek those with support on other platforms. Automatic data-syncing is a huge benefit and multi-platform is going to be a big deal for enlightened Mac and iOS developers.

Already there are some text editors supporting both iOS and Mac OS X. I think this will spill over into most productivity apps: PDF apps, outliners, mind mappers, graphics apps, and any app with a user generated data file. Expect to see a lot of familiar apps on unfamiliar platforms soon.