My mage needs to find 15 more quests somewhere on the continent of Kalimdor. Supposedly these exist, though I've swept every town and questhub on the continent. I know where to find a few of them - 2 for finishing the Timbermaw grind, a couple in Dire Maul, there's the very end of the Dartol's line that I'm not turning in unless it's the only way since I don't want to lose the furbolg rod, and there's the other side of the centaurs in Desolace, though those guys really hate me because you lose 100 rep with one faction for every 4 rep you gain with the other. On the other hand, I've completed a decent chunk of the raid and dungeon 2 questlines, which, by some accounts, should excuse me from some of the more onerous activities.
Retrofitting old content
Say what you will about the differences between WoW's achievements and Warhammer's Tome of Knowledge unlocks, but Warhammer's system gets a big advantage for having been in place at launch. Adding in these achievements for the old world has caused a real phenomena in the game, documented in this comic.
Most of the attention for the WoW Loremaster achievement line has been focused on Kalimdor; Outland and Nothrend have zone-by-zone sub-achievements, so you know exactly where to look, while the Eastern Kingdoms has way more quests than are needed for the achievement. Kalimdor, though, requires very close to the maximum for the Alliance side, which means that it suddenly matters how many quests don't award credit for the correct (or any) continent, etc. It's all very inconsistent - one major questline from the Shimmering Flats awards Kalimdor credit for quests done in the Eastern Kingdoms as payment for the favor that the guy in Kalimdor wants, while the infamous Rakinishu, Demon questline from the Blasted Lands awards a chunk of Kalimdor credit even though you're doing it at the behest of the ghost in the Eastern Kingdoms.
The UI is lacking
The root cause of the problem is that Blizzard didn't go through and do the job properly, because they simply don't have the time to wade through thousands of quests and make sure each step of each one is scored for the correct zone (perhaps accompanied by sub-achievements). The symptoms, though, are a mess.
One thing Blizzard has done to improve things is implement a low-level quest tracking system. The bad news is that the system does not distinguish between repeatable and non-repeatable quests. The worse news is that some repeatable quests do count for the achievement, while others do not. But the real kicker is that there's no reliable way in game to tell whether you've done quests that start from item drops. The item won't drop again to let you know that you've already completed the quest when you click on it. In many cases the quest NPC's at the end of the line don't in any way acknowledge that you did whatever it was for them. So, you're just up with your memory of what you may or may not have done four years ago.
There is a design question as to whether the achievement itself is too hard, but, beyond that, the real problem is that it's very hard to go back and retrofit content without having side issues like this. The achievement system on a whole remains a good addition to the game, with nice incentives to tackle out of the way content. Still, little frustrations like this one, which I'm probably going to park until after I'm level 80, definitely detract from the overall quality and fun of the system.