It really is. The graphics are better. The gameplay is better, like the map where you have to find the gas cans and fill the car to escape. Also the spitter in L4D2 allows you to prevent the survivors from camping all in one corner which L4D1 suffers from time to time.
And yet I can't get into L4D2 as much as 1. I still play 1 and I love it, but 2 I just can't get into it. Not sure why, maybe because I'm unfamiliar with the maps, maybe because it's just a bit too hard for the survivors to make it all the way, maybe just the teams I've been on. I dunno what it is but I like 1 a lot more than 2, even though theoretically 2 is better in every way. In practice, it bores me, and I can't quite put my finger on the reason why. I can play L4D1 for hours and hours match after match, L4d2 for one match before I get tired of it.