WoW is Blizzards first MMORPG.
Blizzard's games are extreemly popular.
Now, ATM, WoW is the fastest growing MMORPG on the market.
So expect some problems with servers.
Oh yea, each realm probably isnt just one computer. So think about how many realms there are and how many computers there must be powering them. . .
Some people I know have trouble looking after just one computer, just imagine the trouble that the blizzard employees are having keeping these servers running.
Couple this with irate customers who like to rant and rave at every small problem, you can start to see that Blizzard are not having an easy time of this.
BTW, I play on the Aszune server. That was down all last night. I hope that whatever the problem is with the server, is fixed now. . .