Don't immediately rule out collection quests. As stated above, some lead to awesome quest chains.

Secondly, some collection quests are better than others.

For instance, there is the human skull collection on in Hillsbrad (for Horde at least). It's 30 skulls. Now, 30x5 seems like a lot, but there are tons of humans there. And most multiboxers can aoe grind those humans. It is worth picking up the skulls if you've already killed them.

Then there are the horrible collection quests. Arathi Highlands has one where you must collect 12 raptor hearts from Fleshstalkers. It drops about 30% of the time, and there are limited number of mobs. And there are usually other players doing the quest as well. I skip this one even when I'm solo.

Look at the drop rate vs. number of mobs, and throw in the quest chain factor.