L4D has a great feel, and really involves you in the game. L4D2 isn't as good. I've spent 1000+ hours on L4D, and only about 150 on L4D2 for this reason.
it's probably because L4D and L4D2's gameplay is so similar, you already got bored on the first and the second didn't hold your interest
I still play L4D, I never got bored of it.