I haven't played it yet, but I know why a lot of people say it sucks.
Infected can't outright maul the survivors.
As in, a lone hunter can't take out a survivor, you can only do a portion of damage. The point of the infected isn't to kill the survivors in combat, it's to heckle and grief them and cause damage that adds up and adds up and adds up and eventually wears them down. It's to cause chaos to break them up from each other so that it's easier to cause damage.
People were expecting it to be a TF2 equivalent, where the infected would be able to maul survivors if they were good enough. This isn't the case, and that's why they don't like it.
Now, that's all conjecture based on the reasonings and whinings I've seen around the internet ("wtf the infected can't kill anything", etc...) but I'll tell you what I think when I play it.