I've dreamed of having my main, dwarf shaman, to be able to sport tatoos since they introduced dwarf shamans back in cata prepatch, and now, after all these years, I can finally make my character look the way I always wanted him to look. That alone makes me extremely hyped about this expansion!
Other than that, I'm looking forward seeing Kael'thas again, maybe giving him some redemption arc, and all the zones look innovative and present us with new, fresh thematic, so that also is something I can't wait for.
I admit that no new class/race is kind of a bummer however.