Left 4 Dead 2 is astounding. There's a lot of fun to be had with just the two chapters, and the gameplay has been improved tremendously from Left 4 Dead.
The special infected all have synergy, even with the 3 new additions. In chapter 2 of The Parish, we were about to go around a corner, but a Jockey attached itself to Nick and ran him off around the corner, a spitter spat on the ground to separate us from him, and a smoker yanked me in the direction opposite of the way Nick was going. Yikes. That attack alone downed 2 of us, and that's just one combo. I'd hate to see a boomer, jockey, and a charger together.
The new changes to the inventory are excellent as well. There's an element of choice when I'm deciding if I want 2x pistols or a guitar, which is a great move on Valve's part, since you can never anticipate which of the two you will need later in the level. The same applies to the Defibrillator and Boomer Bile, since they both also have their own uses. I don't think I'd ever take adrenaline over pills though. I can see where it could come in handy, but adrenaline is too much of a niche thing for me to consider carrying around.
Overall, Left 4 Dead 2 improves virtually every single aspect of the original. I can't wait to finish The Parish and the other 4 campaigns C: