So far I haven't even gotten very far into Vash'jir on my Paladin, but I'm loving every minute of questing in it. The zone is so beautiful, the quests have a great story, and I get to ride a seahorse!
Art is the first thing I list because there's no other zones that, in my opinion, compare to Vash'jir. The underwater scenery is just so beautiful I've caught myself just staring at it until I notice I'm getting attacked. This is the first zone to come along since Nagrand where I wouldn't mind being stuck for an extremely long time, just because I love the scenery so much. The atmosphere is also amazing, I really get the feeling like I'm underwater.
The quests are fantastic as well. I won't go into details but there's the over-arching story connecting events together where, at least for the first part of the zone, the theme is survival. I love that, I love that in the story we're survivors and fighting for our lives. It makes me feel like a part of the events in the zone, in a way that I didn't feel when I played Hyjal in the beta. I think Vash'jir may be one of those rare zones that I won't mind doing over again, just because the story is that cool.
And the seahorse is just plain cool. I so wish I could bring him out in any underwater area.