Having read the http://www.rockpapershotgun.com/2009/06/01/left-4-dead-2-exclusive-rps-preview/#more-12370 article and watched the trailer, I have to say L4D2 is definitely sounding like an improvement over L4D. More weapons, at least two new enemies (one which I have to say I recognize it's function being suggested by a poster on the Steam forum), new challenges, new characters and a new setting all sound extremely fun and worthwhile.
However, like quite a few folks before me, I'm rather dismayed by the fact that all these improvements are in a brand new game rather than an expansion to the existing game. Like people have said, it hasn't been an entire year since the first L4D came out, why are we seeing a brand new game? Online multiplayer games are expected to have longer replay value than this before you have to buy another one (at full game price, I might add).
Another issue I have is that we're saying goodbye to our current four main characters. I think I can safely say that Louis, Francis, Zoei and Bill have endeared themselves to us with their personalities and quirks; Louis with his forehead-slapping moments of optimism, Francis with his grumbling about how he hates this and that and so on. Now I guess one could say that after being rescued you wouldn't expect them to wind up back on the front lines again. However, considering that in L4D they got rescued at least four times, it's not too hard to imagine that whichever the last one was supposed to be chronologically got screwed up like the previous three did and now the four find themselves stuck in the deep South, meeting up with another group of immune survivors trying to reach safety. If eight seems like too big a crowd, consider the last time you played TF2; with more monsters and bigger maps, it could still work.
In all I think I'll enjoy L4D2 when it comes out, but I'm still going to be disappointed that Valve felt the need to axe (no pun intended) their existing characters...and make me shell out more money.