I think Blizzard needs to rework their set up of allowing players to a boss in a dungeon for a mount (Stonecore, Zul Gurub) without having completed it. The system should work so that all of the bosses have to be cleared and if so, the mount has a chance of dropping from the last boss.
It is annoying, to say the least, to be pulled into dungeons only to have people suddenly drop from the random when the boss that drops the mount is down. The rest of us are left to either decide if it's worth waiting a long time for a group to reform or not, and often people are not happy to be pulled into a random dungeon where the bosses are mostly down.
If there is going to be a mount dropped from a dungeon, it should be an incentive offered for the completion of all bosses first.