Do away with tabs, or at least provide an option to toggle on and off. I will
strongly vouch for off to be default. When you have 4 items open in gedit and
you want to find one quickly, the window list entry for the window you are
looking for will often be completely the wrong document and will throw you off
the trail! I'm sure I don't need to attach a screenshot :P

Other information:
This also needs to be tackled in Gaim and Epiphany. Opening links in the
background is largely the only reason tabs were popularised in Firefox, Galeon
and Opera. If you could open links into a new window, whilst keeping the current
window unobscured and active, then the problem is solved without messing with
people's document grouping.

Besides, the window list has a perfectly good (lol) grouping feature, does it
not? :P