Vashj'ir's story is engaging as hell. The slow realization that your faction has stepped in the middle of something much bigger and much scarier than you ever thought was a great reveal. The way you experience story was amazing, too, with scenes from the Naga Battlemaiden's past, the corruption of the sea ancients, and the constant retreats and pushes by the Earthen Ring and Erunak.
The real triumph of the Vashj'ir storyline is that you begin the zone with one confined outlook and, by the end, the very plane of water itself is in danger and you are one of the only brave adventurers who can foil the naga's plot. I sailed to Vashj'ir with the promise of capturing crucial land for the Horde (or defending it for the Alliance), but left feeling like I was instrumental in stopping Lady Naz'jar and her insurmountable naga army that flattened the kvaldir and imprisoned Neptulon himself.
This all comes with a caveat, however. Vashj'ir feels too long. The story comes about at a decent pace, but with the travel involved and the "go here, stop for a while, continue" quest structure slows the pace considerably. That's for the next section, however.
Alex: Vashj'ir manages to do what Hyjal didn't: it shows you that your enemy is powerful and should be feared. I was pulled right into the story of Vashj'ir because things were just going wrong. My ship was sunk, the Alliance is in danger, and the naga were enslaving the gods themselves to do their bidding. I was stuck in a bad situation and had to find a way out (that didn't involving hearthing back to Stormwind.)
It was very cool seeing the naga as villains a way that was more personal than we had previously. The naga played a big role in The Burning Crusade and we knew Lady Vashj from WarCraft III, but they were largely still faceless villains in that expansion. They existed in the world and you were sent to kill them. I'm not complaining about that, note. I liked Serpentshrine Cavern in that expansion. Cataclysm just let you know who you were fighting a bit better and that was really cool. They were hunting you down, making the experience more personal, and you got to step into the ... flippers? of the Battlemaiden and take part in the victories that brought the naga to where they are. It made them relatable.
There were a few unfortunate lulls in the story, almost always because of Budd Nedreck and his merry band of doofuses. I have no idea why Blizzard keeps using those guys. They're the worst. Sometimes the rest of their group is tolerable, Adarrah was strangely endearing with her endless string of accidental sexual innuendo, but in general ... if you're thinking about using those guys, just don't.
I'll echo what Mat said, too: Vashj'ir runs longer than it should. It overstays its welcome by about 30-40 quests. Hmm, maybe if Budd wasn't involved ...
Questing beneath the seas
: The zone had way too many "pick X, Y and Z things up off of the ground" quests for how visually saturated the zone is. It's all big splotches of bright colors and beautiful distractions and tiny little living details, and then it asks you to find the small sparkling object amongst it all instead of letting you actually appreciate your surroundings. Those quests also created a horrific bottleneck when the game was released. One of the very first quests in the zone was to collect items off of the ocean floor. All that accomplished for a starter zone was to cause dozens of players to become congested in one little spot desperately trying to grief each other so they could get the helm spawn and not somebody else.
The zone had a lot of good to balance that out, though. The Battlemaiden quest chain is something that I feel everyone should do, it's that fun. It's one of those iconic Cataclysm
quest chains for me. The Nespirah
chain was also very cool. "Wow, I'm fighting inside of a living creature" is a fantasy/sci-fi stock scenario that we've seen used in raid encounters like C'Thun and Yogg-Saron, but never in questing zones. It was neat interacting with this ancient god of the sea from inside of it.
: For me, quests in Vashj'ir only fell into two categories: fun and engaging or completely forgettable. I loved the epic quest lines and events like the Battlemaiden story and fighting inside Nespirah like Alex already commented on, as well as anything having to do with fighting a losing battle deep beneath the sea. Any time I came upon a mini-hub for quests I would get severely disappointed because I had to break my stride of doing a cool thing for the Earthen Ring to help some goblins or Budd and his crew. The final leg of the journey, where you enter the submarine cove and begin the final assault on the rift was amazing, but punctuated in the beginning with quests that required too much travel across the ridge. Going back and forth 2 to 3 times was not bad but it certainly did not help the flow or pace of this part of the zone. However, getting back to the ship with a crazy mindbender on my head and going insane on my allies was pretty fun.
I think the major difference in quests between Vashj'ir and Hyjal
were that we instinctively knew, for the most part, what was coming next in Hyjal because we could physically see the zone. The quests in Vashj'ir played off on the unknown, the fact that our visibility, both figuratively and literally, was much shorter beneath the waves. For the most part, that stuck and worked, and I'm grateful for those design decisions. I just wish that the zone had less "gotcha" quests when you were trying to work on one thing and were forced into helping other strangers when there was something more pressing at stake.