@Tomkitten,
Sorting by popularity makes the most useful ones (as defined by the most widespread use) prominent. If we sorted alphabetically one user could fill the homepage with aa, aaa, aaaa, aaaa etc in 5 minutes.
I've considered sorting by popularity, then sorting each page alphabetically, but I think ultimately this would be very confusing, as it wouldn't be immediately clear what we are doing.
That being said, the tag search is your friend, and one of our outstanding tasks is to add it to the homepage.