Whatever you do, DON'T go into a red portal. Anywhere. Also, don't pull lone switches in the Wizardlands; you will die. Besides the small bits of new random BS, this is a fantastic expansion and adds a ton of depth and exploration. The new skill trees are very unique and creative, creating new strategies and generally changing how you play. Encrusting is fantastic too letting you add stats to weapons, armor & accessories with your crafting trees. There is a bit of randomness to the effects you apply so be careful with how many you stack per item.
CotW is fantastic and should be purchased by anyone who enjoys the base game. $2.99 for a new crafting system, 6 skill trees, and hundreds of new, unique dungeon floors? Sounds like a good deal to me.