As we said last week, things like reputations and quests are easy to track -- other than dailies, you can't do the same quest twice, so it's easy for Blizzard to know, since day one, which quests you've done. And if a quest takes you to kill, say Onyxia, and you've completed it, then obviously you've got the achievement. Where it gets a little fuzzier is if you haven't done that quest, but you have killed Onyxia -- Blizzard probably doesn't even know yet if they'll do things like look for specific loot from certain bosses in a character's history, or if they'll extrapolate things even farther -- if you're Exalted with Hydraxian Waterlords, does that mean you've cleared MC?
But Tigole does say that Blizzard is making a conscious decision to be as inclusive as possible on these old achievements -- whenever they can, they'll reward credit rather than making you go back to redo old content. That's a good choice -- it does mean that a few people will get angry for not getting the achievement because they killed the boss but didn't do the quest, but the alternative, awarding no retroactive achievements at all, would be even more frustrating.