Text rendering in Windows is significantly worse - sometimes it's blotchy (random characters seem to have more weight than others) - particularly here on HN, and inter-character spacing is inconsistent; at others, the anti-aliasing looks overdone, and text looks blurry and over-smoothed.

I had Firefox 3 configured such that the main menu, URL box, navigation buttons etc. were all on the same toolbar - the menu bar. In that same configuration, Firefox 4 looks somewhat ugly - there's little space between the bottom of the menu bar and the page content (I also use tree-style tabs).

Apart from how it looks, and how it renders text, it's nice. The resizable gripper on multi-line text boxes is nice - that works well here on HN.

Edit: after disabling hardware acceleration, the text at least is much nicer. I don't notice any loss in performance in simple scrolling etc. with it disabled either.