The new icon looks gorgeous, except the 16x16 version, which looks quite funny
Some pages perform much better, namely Yahoo! Mail and many of Google services, except GMail, which seems as fast as always. Even Facebook seems to work faster, although the real test will be using Facebook Chat and see if it still brings the computer to a halt like before. In the other hand, the comment system in /. keeps freezing the browser. Oh well.
Also, it may be my computer, or DropMyRights not playing nice with Firefox, but a cold start is significantly slower than with 3.0. And when I say, "significantly slower" I mean it. A warm start is much faster than before, though.
I'm eager to see sites taking advantage of the new core features, especially <audio> and <video> support, even if the standard is not completely finished. Bye, bye Flash, I won't miss you :-P