Shellyw has a good question on the forums: what quests do you always skip? Blizzard is pretty good about the rewards system -- even if a quest is a pain to get through, they'll usually make the reward worth it. But there are quite a few quests in the game that just don't have anything you want, and just aren't worth the time putting into them.
Green Hills of Stranglethorn is mentioned early on in the thread, and I couldn't agree more -- any of those collecting quests I usually just give up on, as it's usually easier to just sell the pages off inside that zone to someone who is actually doing the quest. I often skip a lot of group quests as well, especially while leveling. While the rewards for those are usually very good (and they're often the end of long chains, which means both XP and items that come from them are great), it's often tough to find a group, so they usually sit in my quest log until grey and then get abandoned.
And occasionally, especially when I need space in my quest log for newer quests, I just plain skip anything that doesn't give an item that I need. If there's a choice of quest rewards, and none of them fit my class or spec, I'll just abandon the quest entirely to replace it with a quest that works better for me. What quests do you find skippable? How do you do triage on quests when your log fills up?