Everything in life is a learning process, and we have learned a lot from every in-game event we have done as to how they can be improved. I'm not going to give anything away, but the player 'clumping' that you have concerns with is something we're specifically attempting to avoid.

The re-opening of the dark portal is an event that will change Azeroth as we currently know it, and it's only fitting that the tremble of such an event is felt throughout the world. It's still likely that players will group up at the portal, but the event we're planning won't encourage it as the AQ event did.

Via WoWForums