I was reading J's Cache's archives, and I was struck by something interesting. In Mount Hyjal, the last quest chain you start actually LOCKS you out of gathering in the south-east corner of the zone. The second quest in the chain is Graduation Speech, and for any toon that's a gatherer, you DO NOT WANT to complete it.
There were at least 10 nodes of Obsidium ore there that my DK Pallormortis can no longer obtain, since he did all the quests in the zone. The only benefit you get from doing the rest of the quests in the chain would be XP and gold (duh), and some ilevel 272 green armor from Might of the Firelord. The quest rep isn't enough to tip you over to exalted, you'll still have to run dungeons wearing the Hyjal tabard to get to exalted.
I've only done Hyjal and Uldum with my DK, but I believe I noticed something similar in Uldum. When I was just going there to gather, I seemed to find a lot more nodes on the north side of the Obelisk of the Sun mountain. My memory sucks, so I couldn't tell you when that stopped. Probably after I did most of the Brann quests, but don't quote me on that.
What I'm wondering is......wtf? Why punish people for completing quest chains? I'm all for phasing, and seeing the landscape and structures change as you progress through a zone. I even would like to see more nodes open up as you help clean up a zone, as a reward for doing what you're supposed to do in the game. This basically rewards you for being a half-ass who leaves zones as soon as they ding high enough to go to the next one.
If you think about it, it even rewards gold farmers (the real ones) who powerlevel to the max level as quickly and cheaply as possible, then spend their time gathering to be able to sell to players and the AH.