Both Kopete and Pidgin fail in various ways -- Kopete because its protocol stack is buggy and incomplete, and Pidgin because it has an amazingly retarded UI. (To be fair, Kopete has its own annoying UI issues, but Pidgin is by far worse IMO.)
I *hate hate hate* Pidgin's away message handling, and I am no Gtk fan. If I could cut-and-paste Kopete's away-message handling into Pidgin, and port Pidgin to KDE, that would be pretty close to my perfect IM client. I really like how Pidgin's buddy list is done, and the fact that they fudge /me support into AIM, etc.
Kopete, on the other hand, has really crappy support for available-messages (AIM and Yahoo don't work at all), and has a problem decoding/encoding AIM's pseudo-HTML correctly (line-breaks, extra spaces get lost on the receiving side, and until recently, extra CRs were getting sent on the sending side -- they finally fixed that bug, *again*). However, I really like Kopete's chatwindow themes.
I just need AIM, Yahoo, and Jabber, along with full away/available message support and file transfers. Don't care about anything else (e.g. MSN, Video, voice chat, etc. -- all of which Kopete seems to be focusing on nowadays). I'd like a GUI client -- preferably Qt, although I'm OK with something else as long as it works.
And don't suggest running Trillian in Wine. That's a cop-out. :p