Despite all the negative response to this announcement, I think that Left 4 Dead 2 sounds great.
Not only do we get a new set of characters in a completely new environment, it will be vastly different in particular ways.
1. Daytime
This will affect many things in-game, specifically that the witch will walk around and be in infected crowds, which will make fighting infected MUCH more chaotic.
2. Melee Weapons
I for one have always wanted to use a chainsaw in this game and this wish shall finally be fulfilled in Left 4 Dead 2. As well as frying pans and axes, these melee weapons will no doubt destroy normal infected and apparently kill all special infected except for the tank in one hit (excluding special infected we don't know about yet, such as the charger).
3. More diverse infected
I'm sure we've all come to know the different looking normal infected in-game, but according to in-game videos, there will be a lot more black infected, as well as much more variety in clothing, as well as these infected in haz-mat suits, whether they be special or normal infected, I'm not quite sure yet, but it looks interesting.
4. New Special Infected
Now we only know about one of these, The Charger, who looks to be a much faster version of the tank although not as powerful nor as long-living. But Valve has confirmed that there will be more special infected to come.
5. New types of gameplay
In some maps there will be variety as to how the map is setup (in particular the maze/hedge looking area in some of the new videos, by the way the videos I am talking about are on IGN).
Also, the crescendo events are going to be mixed up a bit in that you cannot just wait in a corner until the event is over, some shall require you to push a button to end the event, or are made even harder by a parking lot FULL of cars with car alarms during a crescendo event, which sounds INSANE to me if you consider the wandering witch...
Finale's shall also make a change as one of them has you battling across a bridge to survive rather than waiting for rescue, with different paths.
6. New Guns
Now I think we all know that there's only five guns in Left 4 Dead, and while there may not be a new gun TYPE, persay, there shall at least be more variety in weaponry, with a scoped machine gun, and at least a different type of shotgun from what I've seen.
Not to mention, in the end Valve has stated that they're not abandoning the first game.
I personally welcome the addition of Left 4 Dead 2 and will gladly pay whatever price Valve decides for more enjoyment akin to what I have received from Left 4 Dead already.
Source:
http://pc.ign.com/articles/988/988161p1.html
