While burning a stack of xmas CDs I ran into the one and only improvement I've noticed so far since upgrading to iTunes 6:
Now when I tell it to burn a disc, iTunes takes my insertion of the disc as approval to go ahead. The workflow used to be a multi-stage process that included a subjectively interminable wait while it checked out the disc before asking for a final okay.
Sometimes it's a few seconds here and there which make all the difference between a tolerable UX and a maddening one.