When arriving in Dalaran, do not do the quest Learning to Leave and Return: the Magical Way. Also, avoid turning in the quest The Magical Kingdom of Dalaran.

The first quest in the chain is offered by mages from around Northrend and provides a convenient way to get to Dalaran at level 74 and above. You can hearth at any inn and use this quest to quickly get to the city. If you complete either quest, the first quest becomes unavailable, and you lose access to this convenient shortcut across the continent.

So if someone portals you to Dalaran, don't do the quest in the room with the floating stone triangle! You'll be forgoing a great traveling convenience.