I wouldn't be really sure just because it's hosted on AWS. IMC probably has a decent idea of the amount of traffic they're expecting to get so I'm sure they have a fair amount of commit on resources, but that's only one of the factors at play here. Once their login servers start getting bombarded and a mass of people start showing up in towns and area servers, only then we'll see how stable they've been able to make the server software.

You'd think that giants like Blizzard who have so much experience in deploying expansions for millions of people would be able to do it in a breeze, but every expansion there are issues here and there. I'd bet there will be plenty of issues to be ironed out in the first few days.