The major appeal of Gravatars is their universality. There should be no reason why any site can't use them. Having your own functionality on your site is just pointless(Facebook excepted), except of course if it's BuddyPress, in which case the upload and crop feature that is now fixed is awesome.

For those that follow plugin releases, I released a gravatar related plugin lately, and I was amazed at how low the interest was. I thought people would be clamouring for it, but apparently it's just not mainstream enough...

WordPress is world-renowned for blogging. If you were to start another project, along the same lines as WordPress but not to do with blogging, what would it be?
Synchronization, or some sort of contextual text analysis for grammar and spell-checking. Maybe something with anti-spam again like Akismet.