A forum thread like this one is a huge help, but still, there's no way in the game to really go back and easily find which ones we've missed. Even with a list like that, you might spend twenty minutes trying for a drop before realizing you've already done that quest. Blizzard promised us a little while back that they'd be changing the "discovery" mechanic (so that we'd be able to see on the map which areas we hadn't discovered for the achivements yet), and an option like that might be helpful for cleaning up old quest -- say that low level quest tracking might work over the entire map, or there might be a magic box in Dalaran that would have whatever quest items we might need.
The good news here is that Blizzard has built a fair amount of leeway into the quest achievements -- you won't need every single one to get the points, so the more obscure drop-based quests can probably go undone without worry. But just like the World Exploration achievements, a little more help finding the quests we might have missed would go a long way.