Battery life decreased, computer would get a lot hotter when watching youtube, and one time itunes wouldn't close and the computer wouldn't shut down.
Went to the genius bar yesterday evening and had them do a fresh, clean install of Lion. Feels better now, although I still think my battery life decreased.
I think it's simply that things aren't optimized with Lion yet. CNET did a performance test with Photoshop and video rendering, and it was almost always a little bit better on Snow Leopard, since those programs are currently optimized for Snow Leopard and not Lion. Hopefully we will see things being designed for Lion in the future.
Overall, I like the full screen apps, the new Mail app, the new Andromeda galaxy wallpaper (haha), and the new login screen.
Lion is definitely not a new OS. More of an update to Snow Leopard (at least that's what it feels like). Not sure if I think it's worth paying for, but if your computer qualifies for the Up-To-Date program, it's not a bad idea.