Well, a new build came out in the last couple days. As far as I know, no new zones were opened up, but a couple of specs (including my main spec) were made available.
Blue is new, Red is still unavailable, and White is stuff that was already in the Alpha.
Death Knight: Blood, Frost, Unholy
Demon Hunter: Havoc, Vengeance
Druid: Balance, Feral, Guardian, Restoration
Hunter: Beast Mastery, Marksmanship, Survival
Mage: Arcane, Fire, Frost
Monk: Brewmaster, Mistweaver, Windwalker
Paladin: Holy, Protection, Retribution
Priest: Discipline, Holy, Shadow
Rogue: Assassination, Outlaw, Subtlety
Shaman: Elemental, Enhancement, Restoration
Warlock: Affliction, Demonology, Destruction
Warrior: Arms, Fury, Protection
So they actually added more specs this build than remain unplayable, which is a good sign. Given how involved the artifact quests are, I wouldn't be surprised if they're the real hold-up on testing the specs.
So we're now up to 32/36, which is pretty close to 90%. Also, I believe that this means that all the specs getting really significant re-works are now playable (not to downplay the changes coming to the still-missing specs - and Brewmasters are getting somewhat broad changes to be fair.)
In terms of dungeons, I don't think Maw of Souls is officially on the Alpha yet, but they did have a showcase of it with some hand-picked teams from the streaming community, and it looks pretty damn polished (missing a cutscene or two.) Also, Maw of Souls looks AMAZING. There does seem to be a map that might be for Eye of Azshara, but I don't know if it's open yet (also, Azsuna is not, I believe, available, which might be why they aren't ready to have people running dungeons there.)
If I'm remembering all the launch dungeons, that means that we've got three more to go - one in Azsuna and two in Suramar, which are the zones that are not yet playable.
At the earnings report, they claimed that they expect to release the expansion during the summer, after the Warcraft movie comes out. While I'd hope that Legion releases before June, they might be hoping that the excitement over a new expansion would help generate ticket sales for the movie (the opposite might not do so much - if you just plunked down your cash for a new expansion, you probably want to spend your free time playing rather than going out to the movies.) So I think it's very likely we're going to see the movie come out before Legion, though the pre-launch even might start before it.
While fragmented, what we're seeing in the Alpha looks better than a lot of stuff I've seen in rather late Alpha stages. June is four months away, but I don't think that it will take another four months of testing to be ready for a launch. I think the real consideration at this time is when it would be most advantageous to launch in comparison with the release of the film. The movie comes out June 10th, and I'd expect that if Legion hasn't already come out by then, it'll come very soon after - possibly the same week.