Aside from the broken quests or quests that won't clear, the quest log in <img src="/ubbthreads/images/graemlins/div.gif" alt="" /> is really one of the better ones. <img src="/ubbthreads/images/graemlins/up.gif" alt="" />
My suggestion for improving it is to add tabs, for example red blue and green ones, that players can name and move quest trees back and forth on. That way if someone wants to organize them a bit differently they can. Say all the ones down in this area on one tab, or a tab for all the quests I don't wanna ever see again. Or whatever. It would give players enough flexibility so that a "broken" quest would be less of a concern. Of course there will be no broken quest in <img src="/ubbthreads/images/graemlins/beyond.gif" alt="" />, right?