Server teams are made up of the people who write and maintain the code that makes servers go, as well as those who keep that code working. Twitter, Facebook, Instagram, Amazon, Google, and every other service in the world have one or more people in this role. When things go right, nobody notices, and they get no praise. When things go wrong, their phones ring at 3 in the morning, they're up fixing a new issue, and they're answering calls from every cog in the corporate ladder who's screaming about how much money they're losing. It's a thankless job.
Diablo 3's launch has had a number of issues around load. When you have millions of people swarming on a server, ruthlessly trying to log in every few seconds, it causes a huge amount of load. At this scale, things you expect to go right suddenly break in strange and unfamiliar ways. No amount of load testing could adequately prepare the server team behind Diablo 3 for firepower of this magnitude. The way you fix these kind of issues is to look at what's slow, fix it to make it less slow, and hope it works. Do this until load stops being a problem. Oh, and you have to do it quickly, because every second that goes by people are getting more and more upset. And you can't break anything else while you do it.
The people who work on the servers for services of any decent scale cope with new problems every day around keeping the thing alive and healthy. The Diablo server team has been moving quickly, solving issues of massive scale in a short time, and getting the service running again. Players notice and yell the service when it stops working, but the response to it and the maintenance of it has been quick and effective.
Next time you're using an Internet service, or playing a multiplayer game, think about the people who keep it running. If you know any of these people, tell them thanks. They're the unsung heroes of the Internet.