There were just as many bugged qsts and other bugs in wrath and bc. Most of the time it's npcs missing or quest chains not linking somehow. It will all be fixed eventually, there are just other things a lot more important than quest achievements that probably need their attention right now.
As mentioned Mount Hyjal, Uldum and Twilight Highlands are WAY above the Quest Achievement limit, not just a few quests over the limit but tons of them.
As for Deepholm I reached 115/125 and this is without doing any of the dailies associated with the zone which I have started doing now. You get around 7 dailies and some of them has a random quest each day so they can be different so I will assume there are 10 of those atleast so you can hit the Quest Achievement for that zone.
For Vashj'ir I don't know though seeing how I just started that zone.
As far as I can tell, the only zone that I am unable to complete is Vasj'r, missing a whopping 17 quests, and I've done EVERYTHING there.
Uldum I'm going to get the achieve next time I log on, Twilight Highlands is really buggy, so I kinda got frustrated after like my 5th broken quest and just went back to Uldum, which feels like the most complete zone to me.
Deepholme, I did about 50% of the quests, unfortunately I dropped a chain that I had picked up from killing something, tried killing the thing again and I couldn't get the quest, so I'm bummed there. Stuck.
Vashj'ir seems to be fixed, I had two bugged quests before the last server maintenance and was at 142/150. Logged on today and got Sinking into Vashj'ir without having done the elite drop quest from the Giant in the first area. So it's definitely possible to push past the 150 required now, finally.