On top of my head, I can think of a few things:

1) Having a command to remove all, but the most recent version of
installed gems (I had to clean up about 10 versions of Rails,
ActiveRecord and ActionPack the other day)

2) Having a "show" command, where there would be a detailed explanation
of what a particular gem does ("search" could also search that
description), a URL, an author email, etc.

3) Already mentionned by Gabriele, a cute progress bar (using
progressbar available in RubyGems)

Vincent.