I've not noticed any new/more speed on my MacPro, or on my girlfriends macbook I noticed it connects to Airport Express faster, but that's it.
Quicktime X is just had a makeover, but it's no good to me as I have Quicktime Pro 7, and there isn't any A/V controls in Quicktime X, but I was pleased to see it was still there (hidden in Utilities for some weird reason).
Safari 4.0.3 is playing up, I keep getting the tiny spinning beach ball of doom! -
http://en.wikipedia.org/wiki/Spinning_wait_cursor
It lasts so long, that I have to force Quit!
And I've Reset Safari, but the problem still continues

Oh, and Time Machine is slower, as well!
Both machines are new, so I just did an upgrade.
I was under the impression things were going to start really flying, but to be honest, I've not really seen a difference between SL and Leopard.
I guess it's all under the hood with the "Nehalem" processors (so to speak)?