A DLC could easily incorporate the new zombie types, weapons and maps into the game.
To be honest, you'll find that hundreds of thousands of people agree with me after a massive ragefest on the Steam forum, the sheer number of people who've signed petitions, rallied up etc with the same opinion as mine is staggering, though frankly, I don't associate myself with that lot, because I'm not that overzealous, I'm not going to try and stop them making Left For Dead 2, I'm just voicing my valid opinion, which happens to be that I feel the creation of Left For Dead 2 so early on is utterly pointless.
Oh and, no, if I were a Valve game developer, I wouldn't really care what any given number of people think about my idea, if anyone outside of the company criticised it, called it pointless/stupid/etc, simply because three quarters of Valve's fanbase worship the ground they walk on, Valve could sell crap in a box and people would still buy it, which just relates straight back to L4D2, it doesn't matter what several hundred thousand people think, there are more out there who are still perfectly willing to buy Left 4 Dead 2, and as such, my opinion doesn't really make a bloody difference.
Also, I'm a cynic when it comes to gaming, hype makes me more wary than excited, my policy is to assume everything is going to be a steaming pile of crap until proven otherwise, and that policy has saved me a lot of money on games I would have hated, whereas 95% of the games I've actually purchased, I've completely enjoyed, and the other 5% I've warmed to in time.
So frankly, I would say it's fair for me to criticise a game I haven't even tried yet, because it'll probably save me a nice heap of cash that I could spend on basic living expenses, such as the rent, and strawberry cornettos.