Im always suprised that people seem so quick to blame capacity as the reason the servers are having an issue. From someone who supports large infrastructure for similar applications, 9 times out of 10 its the application that has issues, not the servers or the network.
Considering the fact that D3 has both a client and server software stack involved, and both are home grown, built from the ground up. When you go live with a certain application, there are so many things that can/will go wrong. And as I mentioned above, if you do your hardware correctly, its rarely the hardware that is the issue.
Even when you test (ala stress test) with the public like that, it dosnt always give you a similar experience when you go full production. So I would list it like this: server app, client app, servers and network. The last variable to the list is user actions. You can control the first 4 (and that can be a headache in itself). But you can never fully expect what the users are going to do. Going live with a real money auction house? Hmm, perhaps people are attacking the new app hoping to get some real money loot from a unexpected bug in the app?
So, in summary, yeah, it could be just server capacity, but it could also be a number of other things as well. And more than likely, it oculd be a nasty combination of things (this many people + hacking attempts at auction servers + server bug = fail)
That being said, Id more suprised if there were no emergency maintenance on launch day then to see servers go down.
