The thing about lunging with the knife can be determined somewhat by your actions before you actually hit the Melee button. If you're sprinting forward, you will most likely do a lunge. Otherwise, the slash comes out much more frequently. I always have better success in hitting people with the slash over the lunge, so to make sure I actually slash, I typically hold backwards as I knife.
The knife range is a little bit odd. I haven't really figured it out, but it must have some bit of range to it, because I've consistently slashed people that I would expect not to hit.
MW2's melee system was the strongest, in terms of balance and consistency, but to be honest, I like Black Ops' better in terms of game balance. As much as I miss my Commando. ;_;
