Here's an odd fact: The Mac App Store actually requires that Spotlight be enabled for one to download anything from it.
If it isn't, the App Store just beachballs endlessly until it has to be force quit.
I had disabled Spotlight back in the Tiger days using the great Spotless
utility because I was having issues with Spotlight running for hours on end every night, interfering with backups and slowing my computer down.
Luckily after re-enabling Spotlight (under Snow Leopard), all is well now and Spotlight behaves as it should.
I found this answer by Googling around a bit, but it wasn't easy to find.
Hope this helps someone else.