Considering Chrome's GTK integration is god awful (on my box anyways), I can't wait.
1. Facebook
2. Google Wave
3. Google Docs
4. And a bunch of other webapps I'm forgetting ...
"Well, let's say you can shave 10 seconds off of the boot time. Multiply that by five million users and thats 50 million seconds, every single day. Over a year, that's probably dozens of lifetimes. So if you make it boot ten seconds faster, you've saved a dozen lives. That's really worth it, don't you think?"
Those few seconds add up.
I accidentally discovered a neat DoS in Firefox 4 while working on some JS: the alert loop from hell is persistent even after you close the tab. IIRC, in 3 it was possible to close the tab in between alert()s and they would go away; in 4, you'll have to kill and restart Firefox to get rid of them. Chrome still has an advantage there.