The speed! I still prefer Safari on OS X, but I am very happy with FF3 over FF2 in Linux and on Windows. Zoom is good too, but I honestly haven't used it since I first tested it out... The other feature I notice most often
I did have something that I haven't had time to test out related to the autocomplete thingy... In Windows, on 3.0b4, I found that autocomplete for the URL bar was finding candidates for me (that are sites I've visited) even without these sites apparently being in my bookmarks (unless they're hidden somewhere), and in spite of me having my history set to one day and auto-clean turned on. And this wasn't even installed over a copy of an FF2 profile. So I'm not exactly sure where these autoclean results are coming from, and it kind of concerns me. I guess I have to dig around and make sure that it isn't storing some personal data it's not telling me about (this is on a shared computer).