L4D is badass, it's a blast if you've got a good friend group. The Developer commentary is also just as fascinating as usual, it's incredible to see how the silhouette artstyle used in TF2 impacted the designs of the special infected. Hell, the smoke around the smoker is for readability, you'll always be able to see him and tell when he died. The tutorial system also returned for Portal 2.
Me and a friend group got maybe 100 hours on it, especially due to mods. Good times.

I don't play L4D1 since it's obsolete now, but I prefer some of it's particle effects.