BlizzPlanet has already found a fun little set of Easter Eggs hidden in the fountains around the new city hub of Dalaran -- apparently with a little Fishing skill, you can bring up the wishing coins of some of Azeroth's famous inhabitants. Some of the coins are spoilers (so be careful clicking through if you want to avoid that kind of thing), but others are just in there for fun -- Arugal has apparently wished for his own estate (with some doggies to keep him company), and Attumen would like a much more comfortable saddle.
There are some good references to the extended universe in there as well, as apparently a few characters from the Warcraft novels have made appearances and throw wishing coins in Dalaran. It's this kind of stuff that is going to make exploring Northrend so much fun -- while Outland was relatively unexplored territory, Dalaran and the rest of Northrend are already well-traveled in the Warcraft universe. It'll be a really good time retracing the footsteps of all the characters we already know.