1) Plug-ins like Firebug, web dev toolbar, greasemonkey, and resizeable textarea
2) Tabs are well-placed and readable, and you can simply double-click next to tabs to open a new tab
3) A proper status bar that's always visible
4) A full menu bar
5) No Adobe Flash built-in
I've installed Firefox 4 RC. The colors are loud. Tabs are on top, tab text is illegible, and double-clicking next to tabs doesn't create a new tab (it simply resizes the browser window). The status bar and the menu bar are gone. I couldn't even find Firebug and Greasemonkey.
Luckily, a few of these unfortunate decisions can be reversed. Apparently, you can turn on an "add-on bar" to show the controls for Firebug and Greasemonkey. You can also opt for a top menu bar, which has the added benefit that it re-enables the "double-click to create new tab" behavior.
IMHO, the new UI sucks balls. If Mozilla wanted to copy Chrome/Safari's UI, they've succeeded.
EDIT: After viewing some of my websites in Firefox 4, I must say that CSS3 and text rendering is a lot better than in FF 3.6. From what I can tell, it's on par with Chrome. I just wish it didn't have its UI as well.