We usually don't realize it, but there is actually an upper limit to the number of instances that can be created in WoW at any given moment. If all the players on your server somehow decided to run Karazhan at about the same time, you'd find that after a certain number of raids had gotten started, some players would be stuck at the loading screen, waiting for the others to finish. Normally, of course, it's not a problem, because people don't all go to Karazhan at the same time. But at prime time, the 2-vs-2 arena instances are as popular as can be, so the 2-vs-2 arena instance slots get full, and frustrated players sometimes find themselves waiting a good long time for one of their turn to fight.
Last month, Drysc said that the Blizzard team still didn't know what to do about it. But now, he says that they do have a special change planned to help solve this problem. It's going to require a patch, though, and not the upcoming patch 2.3.2, either, but it is scheduled for some point in the future, "as soon as possible." Patch 2.4 perhaps? "The change," Drysc says, "will make it so that more instances can kick up in a shorter amount of time. Which doesn't necessarily mean there can't ever be a wait again, but it should drastically decrease them." However much "drastically" is, shorter is always better than longer, when it comes to queues!