The first build of the Legion Alpha isn't even really bare-bones, it's more like the bones in a severed hand. Right now the alpha has only the Demon Hunter starting experience, and while that's probably the first thing I want to check out if I get into the alphabetagammawhatever, it does mean that they do not yet have the Broken Isles open (I imagine they're still working on making sure the level-scaling stuff works - that's probably the biggest engineering challenge they've set for themselves with Legion.)
The other reason it's bare bones? Right now it is not available for Macs.
As a Mac gamer, it's easy to be a little sensitive to these things. While it may be a different story for iOS, most game developers basically ignore Mac computers. Blizzard has always been a steadfast supporter for Mac users, but that steadfastness was shattered when it was revealed that Overwatch, their new team-based FPS, would not be made available for Mac.
So a few people have gotten a little anxious about the idea that, if the Alpha is not available for Macs, the expansion itself might not be. I certainly worry about that kind of thing, but I think it's practically outside the realm of possibility for them to release a WoW expansion that does not work on Mac OS. WoW is eleven years old, and for all those eleven years, player like me have been able to do everything that PC players could. Expansions are not new games, and so while technically Blizzard isn't obligated to do anything, it would be seriously, seriously bad PR to suddenly cut off a wide swath of players from a game that survives thanks to established players who want to keep playing the characters they've had for several years.
Overwatch might set a frightening precedent for future Blizzard games, and whether Starcraft III or Diablo IV might wind up being PC-exclusive, but because WoW is actually one game that was released over a decade ago, and more or less stops working if you don't have the latest expansion, I don't think they could actually cut it off from Mac players.
According to the Mac team at Blizzard, they're just lagging a little behind on technical issues that I won't pretend to understand. According to them, the next Alpha build should be available to Mac users.
So I'm crossing my fingers for an invite!