And the ability to block ads, which is possible in Conkeror.
I'd also prefer an Emacs-style keybinding set (or at least the ability to map one), so I don't have to switch muscle-memory mode when jumping from my editor to graphical browser. But, I'd be willing to give that up if the memory footprint of a thin Webkit-based browser is more minimal. I occasionally have to restart Conkeror after sustained heavy use due to it hogging system resources (it'll eventually bring my Atom n270 to a grinding halt). This is probably xulrunner's fault though, I suspect.