D2D/Directwrite will become more of an issue the closer we get to FF4’s release date. It will not be a good idea to subject new users to something as basic as font readability…that’s too much of a comfort zone shock for new converts. Disabling D2D/DirectWrite by default off the bat (corrected with a point release down the road) is a saner approach, because I think Mozilla is underestimating the user pushback once the non beta testers (who generally have more conservative machine setups) are involved.
I guess there is some solace (depending on how you look at it) in that IE9 will probably beat FF4 in coming out first. That should increase significantly the chances Microsoft will fix the DirectWrite issues in the near future.