there are a couple of features which i think are indispensible (for me at least) on an instant messaging client:

multiprotocol
people use a whole variety of messaging clients now (why!!) so i don’t want to have to run MSN and ICQ and AIM and skype and all the other crap. i want one program that’ll run all those things.

metacontacts
i’m tired of having two to three contacts on my list from the same person. the solution to that is to collapse all of those contacts into one meta-contact so you only have each person once on your list.

avatars in list
avatars are a means of self expressions kinda like your nickname is; people change it depending on their mood, the day, etc so i think it should be on the contact list itself (not like as a tool tip). they should get rid of the generic status icons and just show the avatars instead. you can do statuses by colour or something else.