So this leads me to wondering something: how does the application’s data get backed up? Right now within the iOS App Store, your data is backed up when you sync your iOS device and this information is then backed up again when you backup your computer (you do backup your computer, right?). But if your application data is just stored locally and you do have to do a re-install of your computer, even though you are able to download the software itself again, your data may not be easily placed in the correct location, even with backups.
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Syncing is going to be key to apps within the Mac App Store. In order to facilitate the ability for a user to download their data upon a complete re-install, it is going to be necessary to include sync within the applications. This would allow a user to quickly upgrade their Mac by re-downloading all of their applications again from the Mac App Store and then launching the program. Upon launch, the initial download would request credentials for accessing their account and then the automatic download would occur, thereby allowing the user to quickly get back to their last synchronized state.
Such a system would be killer when upgrading to a new computer, but what about syncing data between Macs? Syncing data between my iMac and MacBook Pro is more tricky for some things than syncing my iMac and my iPad. The App Store could solve this pretty easily, I think.