It may seem obvious, but if you have an unknown file extension and try to open the file, you will now be presented with an option to search the App Store to locate an application. I realized this when I tried to open a kdb (a keypass database file) and was presented with this new option.
[crarko adds: This is obviously only available in 10.6.6 and later. Just for fun, I looked to see if Spotlight would search the Mac App Store. It doesn't, yet, but I won't be at all surprised to see that integrated in 10.7.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110119151445374