For the people asking about the counter knife window, it is ridiculously long at this point.
As someone else said, it only happens when knifed from the front, but the window had to be at LEAST 3 full seconds. I have melee on my mouse button so I am not used to using the F key, so I turn around right in the middle of the animation, notice the button prompt to F flash on screen for half a sec, fumble around to find the F key, then spam the hell out of it.
I got the counter. I cannot imagine the window will be this long in the final game. I am sure it is this long so everyone gets a chance to see it and have a "holy **** " moment.
Then again, in BF3 getting frontal knived while dumping ammo into a Terminator-like opponent who just pulls a knife out unfazed by bullets and stabs ya in the throat was kind of annoying. Maybe this is their answer to that: Frontal knives will be an extremely high risk choice.
Still, I expect the window to be at LEAST halved by DICE.