not everything will be perfect, but world design should be finished, so must be quests before it becomes beta to begin large scale test. so many PH quests, and as mention to many parts of zones unfinished.
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I think one thing to note is blizzard seems to be liking the usage of alpha as a flag to manage expectations more. Open beta is likely when they feel it is totally feature complete and maybe just needs some number tweaking and want some stress testing.
It's very apparent that Blizzard devs communicate a lot between their different games. Jeff Kaplan from Overwatch has stated numerous times that he greatly regrets labeling the Overwatch beta as "Beta" when they did due to the enormous out cry of people not getting keys/ only watching streaners, in the gaming industry landscape today players associate beta as early access and open beta too much.
That's why this Alpha is so much further along and I think that is also why they are being cautious as to when they change the name.