I’ve got a few more for ya:

“the tease” – someone who logs in, just long enough for you to open up a new chat, but then logs off just before you hit that initial ‘send’ button, leaving you with nothing.
Sometimes, they’ll even do it two or three times in a row.

solution: wait it out – do not initiate a new chat for at least two or three minutes after someone logs in.

“the stalker” – might not be the best name, but we’ve all got someone like this: you were the one to introduce them to IMing in the first place, and you KNOW they don’t have anyone else to chat with, so when they log in, you know it’s for the sole purpose of chatting with – YOU.

solution: try to get more of their friends in on the fun, so you’re not the only target.

“perpetually away” – folks that never, EVER change their status from “away”. If you don’t want people to bug you that much, frickin’ QUIT IM!!

solution: remove them from your buddy list. If and when they want to talk to you, they’ll pop up again.

“super into it” – folks that are constantly changing their icons and status message. I mean, yeah, I guess it’s cool that you can get really specific and all, but dude – I really don’t need to know when you’re taking a crap.

solution: try not to pay that close attention, or, if your client supports it, turn off the whole status message display thing completely.

My final thought on the whole IM thing is the lack of inter-operability between client software.
When, oh when will they figure out how to get simple file transfer, picture sharing, and god forbid voice and video chatting working on all of the popular alternative clients?
Time will tell.