There were a few things that I didn't like about this game. Of course, because it was made by the people over at Seirra, it reminds me of Half-Life. I didn't like playing Half-Life with other people because the movements were far too quick and unrealistic, and the aim sensitivity seems very high.
In Left 4 Dead, there seems to be a lack of enthusiastic type animations for zombies when they are getting shot, when I shoot a zombie I feel as if my bullets aren't penetrating at all.