The problem with making release date guesses on Mists of Pandaria is that Blizzard is famous for not releasing until they are ready. I have to think that some of this principle will be shed away in favor of postponing some content for future smaller releases and tuning class-specs over and over.
I've seen a fair number of bugs in the beta. I've seen blue and white checkered squares here and there. There aren't names on any equipment. Some class mechanics seem out of whack. That said, I'm surprised by how much does work.
This beta has been problematic for Blizzard. The engineers and QA folks certainly did not want a million beta testers. That sucks. Their only bug reporting mechanism is typed text. There aren't filter chains or built-in ways of narrowing the bug declaration. So the current beta could really hurt them.
They certainly have changed a lot. While we think "OMG they changed all my talents" what they've really done is simplified their balancing. Almost each class has three specializations which provide all the base spells. Then there are modifiers from the talents and generally one or two extra spells from the talents. That is less to balance. And Blizzard wont care too much about balancing things much until level 90.
There is the issue with producing a box, DVD, and physical product ... getting it into stores ... giving them the lead-time. The whole industry production sales, pre-ordering, and stocking process. Blizzard may just screw them a bit and do it entirely on-line. More head-aches of course. But really not much more considering the DVD they'd ship wouldn't be much better than the beta is now and it'd have to on-line patch anyway.
I have seen many release date guesses spanning from as early as the end of July to November. I am thinking they'll go early and iterate with frequent patches. Very early. I'm going with the Price Is Right(R) model and low-balling and not going over: July 10th 2012 is my guess.