The things I need in a theme are:
winamp controls - I'm stuck to those
rainmeter - just to figure out if my cpu has filled up again :P
taskbar - d'oh
clock - I'm a very close clock watcher
systray
Inet icons (not mail, I use msn)
GOOD BLINKING! SOOO many awesome theme's I can't use cuz I can't see if mIRC is blinking or MSN is blinking.
And a good readable font... all the Ashtefere's theme's are a joy for the eye and look better than most themes, but I just can't read them properly :$