While I have no problem with the new environments (including daylight), I do think that these people have a point when it comes to feeling like Left4Dead is being abandoned in favor of the sequel. They promised a whole lot of new content and the only truly new thing we've gotten (not counting the ability to play versus mode on the other levels) was survival mode and the SDK for modders (and the SDK was long overdue).
It seems like Left4Dead 2 will be bigger, but since it's set to come out only a year after the first game with all the stuff they promised they would have been supporting the first game (they promised a TF2 level of support), I think people are within their right to be a little steamed.
Lastly, a big 'lol' to those saying these are PS3 fanboys considering this is a Steam group and upon looking at the members, you'll see that many are playing Valve games like TF2 and Left4Dead.
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Now, after hearing Valve's explanation of Left4Dead 2, I can give them a little benefit of the doubt since it seems like they're going to be tweaking the formula of the game and such. However, I still believe the people upset have good reason to be. Considering Left4Dead is $40 on Steam and has now been rendered obsolete, we probably wont see any more content released for it outside of bug fixes and such. In a way, it's kind of like a middle finger to the awesomeness of the recent free weekend and release of the survival mode.