Anything that was in memory was potentially leaked in a way that can't be traced. Certs, SSH keys, user passwords, database passwords...
Oh, and if an admin logged in at any time during the window in which you were vulnerable to Heartbleed, or if any similar credential ended up in memory in any way during that period - consider yourself rooted.
Heartbleed is a "burn down the server, redeploy, restore database backups, expire all credentials" event, not just an "apt-get update, done" problem. And it hit over 2/3rds of HTTPS websites (ideally: anything with passwords, then some).