The #1 thing I can't stand about the fennec UI is the fact that it blocks out the standard Hildon status bar/menu. I already like having all the status info that it gives, plus I run my own monitor clock plugin for Advanced Clock Plugin, which gives me a bunch of other status info. It bugs me when Maemo 5 programs don't give you the option to use the built in status-bar.

If it wasn't for that, and the lack of the swirl-to-zoom gesture, I'd be willing to convert, methinks.