This drama happens everytime a new Blizzard game launches. Hell, WOW is nearly 10 years old and still everytime a new expansion comes out, the B.net servers come to a screeching halt.
If anything, this should be more reason to be mad. Like you said this happens everytime Blizzard releases stuff, so if anything they should've prepared more for this. They certainly have the resources for that. In any case, the backlash would not have been so bad, if the users had offline singleplayer to fall back on. Sorry to harp on that same point, but its true .
Well, we can also say that the Metacritic rating as of now more or less accurately represents the experience people are having at this point of time. As you said, once the rush evens out and peoples' experience improves, the metacritic rating will come to a expected level.D3 Metacritic User ratings as they stand right now, they might as well be a rating of how current gen TCP protocols fall face-flat when faced with an overwhelming amount of connections rather than the game itself.
I doubt any server farm in the world can smoothly go through a launch consisting of extremely high density traffic.
Just give it 3 weeks, if these issues still persist, then I'd start worrying.
Until then the reality and limitations of current gen technology can't be worked around.