As odd as it may sound, I'm most looking forward to the musical score additions. I've always felt that the in-game music deserves more appreciation than what it gets, as it really helps to immerse you in the range of environments. It can stimulate that calm and serene feeling when you're leveling in the woods, or it can create a heart-pounding feeling of anxiety when you're in a high level dungeon/raid for the first time.
Suffice to say, I was beyond ecstatic when a Blizzard poster confirmed that Cataclysm would be introducing a variety of new compliments to the current ambient soundtrack.