Well, since bugs were primarily that which made me quit the private server, then a bug checker. A good one. Bugs were the main reason the private server didn't expand. I expect they'll be obtaining in one way or another the Cataclysm expansion once it comes out. Again, full of bugs.
One thing I remember was an invisible monster called "OLDWORLDFUNCTIONDONOTDELETE". I had to help my bro, whose character was of the alliance while mine was of the horde, to kill it. Eventually, it died.....then another one instantly attacked. I said "fuck it" and left him to die.