I don't think I've used x11 since switching to Mountain Lion (and thus Xquartz). I recently tried to use it and my key repeat isn't working. When I press a key, it inserts the corresponding character once, then a second time after (very roughly) a quarter second, then never repeats again. I've tried
xset r and
xset r on from both my x11 terminal, and normal Terminal.app --- neither has any effect.