I'll take a simple username and password... profile sync (birthday, avatar, sig, base custom profile fields) would be nice too but ill settle for the basics.
For content, I'd like to be able to display Forum X and all public threads in it on a different site. Linking the users to the profiles (above) would be great, but having it show up with the name under the 'guest' usergroup would be fine too -- just so long as content is syndicated back to the original forum.