Me: What would a hard-mode Ragnaros have been like?
Adam Holisky: Kill Ragnaros using only one tank!
Eliah Hecht: Domo comes back from the dead and starts randomly sheeping raid members in revenge.
Other suggestions included killing Hakkar with all of his priests still up, hearthing with Hakkar's debuffs and infecting at least 500 players with Corrupted Blood (back when this was still possible, of course), killing at least 500 Dwarves without dropping combat in the Lyceum, the Stratholme timed run, and -- as Sacco suggested -- "getting through an UBRS run without (anyone) quitting." Having recently leveled a Shaman through this content, I can tell you that's one achievement I wouldn't have managed.
If you were back in classic WoW again with no chance of advancing beyond level 60 talents and gear, what would make for a worthwhile achievement?