[quote name='secretvampire']This cannot be taken as a rule. I played City of Heroes right after release and never had any problems, it was excellent. And seeing the amount of beta testing they are doing for WOW, I am betting it is going to be a solid launch.[/quote]
Sure it can be taken as a rule. You have to realize that there are only so many people willing to pay to play. Companies today make very few strides towards growing the MMO market. Rather, they look to take the biggest slice of the existing pie for themselves. Consequently, whenever a new game comes out, you'll see its population spike big time, especially for big name games like CoH and WoW. Between 3-6 months, a lot of players that signed up for the gimmicks will grow tired and leave the game, as will many of the powerlevelers and other 'band wagon' players.
In addition, bugs and balance issues usually get their most dramatic fixes early on. No matter how big the beta was, it's a whole different world when a game goes live.
Basically, the whole game experience doesn't begin to show its true form until approximately 6 months after launch. Sometimes it's sooner, sometimes it's later (eg Anarchy Online).
edit: I need to brush up on "its/it's" big time.