Actually, one thing I'll add here - I think the main complaints were from PC players (or it least it felt like it to me).
On PC, a sequel will normally come out say 2 years later when there are newer graphics cards, and therefore it is expected the game will probably ship on a newer or greatly enhanced version of the engine.
Clearly, on consoles, this doesn't happen due to specs remaining static, and while games can be given a good boost (say Uncharted to Uncharted 2) you rarely see the level of change from say Unreal to Unreal 2, or Half Life to Half Life 2 on a console, and if you do it's normally because the first title was graphically weak in the first place.
Now, even on PC Valve have been bucking this trend since they released Half Life 2, sticking to refining Source rather than producing a whole new version of the engine, and I think L4D2 just picked up a lot of concern from PC players that the engine hadn't really changed much, that the community was being hurt too early in L4D's lifetime and so on.
So while I'm fine with the title releasing and will get it, I can understand that the complaints are understandable and not just whining for the sake of it.