So far I've quested in Vashjir, Hyjal and Deepholm and I must say I am very impressed with the new content. Deepholm and Vashjir are amazing and like nothing else in the past six years of WoW. Deepholm especially is visually amazing.
I feel like Blizzard really realized Northrend was bland and boring and stepped it up to a whole new level for Cataclysm. Dungeons as well are awesome and actually have interesting and fun mechanics. Stonecore and Throne are very enjoyable and really stand out to me as well done dungeons.