It's understandable really. Valve should be able to release a new title whenever they want, and of course they can, but with MP focused titles there is normally a bigger gap between titles and you'll find a core set of gamers who feel they're being asked to move on before they believe they've gotten full value from the current title.
I'm a little surprised L4D2 was announced so soon myself, as on PC (so far am I'm aware) in most cases releasing a new iteration of a popular MP too close to the original hurt sales.
Try to be reasonable... its easier than you think...







