Ok I've been discussing this on neowin too but I must say some things I'm very happy with in FF3
Well one big thing really, the awesomebar!
It searches your bookmarks and lists them on the fly so no more need for the bookmarks bar being visible BUT!
I Marmite it at the moment.
ON one hand it's cleaner to look at hidden and gets me using the Awesomebar more but on the other hand usually I like to see my bookmarks listed as I bookmark pages by catagory and part them out using separators (see fig1)
fig1:
[snip]
I'll probably get used to it as you can just hit ALT and display bookmarks normally if you need to whilst all other times it's neatly hidden away and you just use awesomebar.
So, with The Hide Main Menu extension installed my Firefox now looks like this: