In addition to the cluster, Update 5 is including the next book in the epic storyline, The Prince of Rohan. Through it, players will gain a nifty set of Ranger cosmetic armor and participate in an epic battle at the gates of Orthanc. The update also will add an instance finder, unified currency, a LUA plugin manager, a reputation update, and far, far more.
Oh, and there's finally coffee in the game. Mmm... coffee.
Turbine has posted the official patch notes for Armies of Isengard on the site, so you can read up on all of the changes while you're patching the game!