I really dislike L4D2. In L4D, the specials don't appear as often, and you actually freak out when they do.
In L4D2, they pop up so often it's just meh. I think L4D2 lost the charm L4D had. Also, the community playing L4D is a lot better than L4D2.
I also like the L4D characters heaps.
There's also a L4D comic here -
The Sacrifice
Bill is just damned awesome. It really makes the last campaign in L4D sadder than it already was when you understand that Zoey and Bill never made up after their fight.