The Mac App Store is already available in the latest edition of OS X Snow Leopard. You can actually think of the Snow Leopard version of the Mac App Store as a preview. Sure, it works fine, makes it easy to buy software, and provides a one-step download and installation process, but with Lion, the Mac App Store will get a few new features.
Lion will incorporate Mac App Store downloads directly into its Launchpad service, providing a handy way to organize applications and launch them from an interface that's similar to the iPhone or the iPad. You can, of course, continue to use the Dock; items you purchase from the Mac App Store will still show up in your Applications folder.
Although Apple hasn't mentioned any additional Mac App Store features for Lion, it makes sense that the Mac App Store will get a facelift to address many of the early adopter complaints about the service. Expect to see better search and sorting, additional categories, a better way to find new apps that have been added, and a more uniform interface.