For me, it's the lack of addons that can radically change the UI. In Firefox, I run Vimperator (there's a few addons that are catching up to it in Chrome, but they cannot hide the various toolbars afaik), Hide Title Bar (minor, but nice to have), and Tree Style Tabs (essential. Chrome cannot handle large numbers of tabs, and all the addons I've seen that attempt to handle this issue aren't nearly as nice). The UI is almost unrecognizable, but I actually like the set up. Unfortunately, Chrome simply cannot do that type of modification, at least as far as I know. I'd be very happy to be proven wrong here. The speed of Chrome is fantastic, I just wish it had those few addons.