As you have probably heard, Blizzard will be removing the Plagued and Black Proto-drake mounts from the game as of 3.1. Currently they're available from the meta-raiding achievements, but since Blizzard wants to keep them rare, they've put a limit on the time you have to finish off those achievements before Ulduar gear and higher levels make them too easy. Lots of players are less than happy about this -- we're not in the right place right now to do these achievements, but we still really want a chance at these drakes. But Bornakk defends the decision over on the forums: he says that they didn't want the achievements to be "watered down as patch after patch goes by."
One player suggests tweaking them in a little different way: the new Herald of the Titans achievement actually requires you to have a certain iLevel on your gear, so that you can't get it if you're geared out so well it makes the fight trivial. Players want to know why Blizzard can't do that with the Glory of the Raider achievements, so that no matter what new gear we get, the difficulty stays the same.
But Bornakk confirms that it's not a matter of difficulty, it's a question of rarity. If players were always able to get the mounts, there would always be more and more of the mounts in the game. They want to cut off the supply, period. The good news is that there will eventually be more rare mounts to go after (and I can't wait until I see that Gnome head flying around). But the proto-drakes' time is almost up.