Last weekend I finally got something I'd wanted for a while but had never gotten around to doing since I stopped raiding right after we had cleared normal DS! I just didn't have the patience to continue raiding DS heroic as it was one of my least favorite raids. Reins of the Twilight Harbringer from the Glory of the Dragon Soul Raider achievement.
He's fucking glorious!
Are there any achievement mounts you've got your eye on?