I'm excited for the excellent quest chains I keep hearing about in Lore's videos, which he described as more than just an XP grind, but a real storyline feel.
I'm excited for the upgrades to environments (no more copy-paste water!) and the expansion of the phasing mechanic to apply to environments as well.
I'm excited that the Alliance will finally have a race that looks badass.
I'm excited that levelling 1-60 will feel like less of a chore and be more fun!
I'm excited that, because of the expanded phasing mechanic, we may get to see major advancements in the Warcraft story, instead of simply knowing who the enemies were and waiting to be able to kill them.