L4D2 is a great zombie killing game, but sadly it doesn't come off as being anything groundbreaking within the genre.
There is a lot of campy humor which is excellent for this type of game, but due to lack of any real horror in the game, it turns into a comedy of sorts.
Replay ability is also very difficult since the setting is not inviting and it becomes really tiresome performing the same tasks because freedom of movement around the maps are not as vast as in other games. I must compare this game to its rivals, I am not a blind Killing Floor fan boy, but in my honest opinion KF nails the entire co-op horror theme much better than Valve's offering. In KF the entire setting is gritty and moody. You feel vulnerable, and the graphics even though they run on a less sophisticated engine than Source look overall better than L4D2, some shader effects are undoubtedly better in L4D2 but overall KF looks sharper and the recoil and weapons animations and overall diversity of weapons make L4D2 pale in comparison to KF. Also the perk and class system in KF makes for a varied game play, couple this with a better variaty of monsters and you are almost forced to see the flaws in L4D2. This game has excellent production value but it just doesn't pack the punch that KF delivers, and this is why you stay hooked on KF for a VERY long time.
Valve needs to introduce more atmosphere to their co-op horror games to be able to advance the series into a more atmospheric franchise, because the elements are there, zombie dismemberment in this game is the best of its kind, but variation in monster types and some much needed suspense would have made L4D2 a brilliant game.