I've only played the 1st campaign so far, but it's definitely been amped up.
In addition to the fact that you have a lot more weapons and items to use, the game really cares how many players are in the game. I played with 3 other people and in that one campaign we killed over 2400 zombies!! That's compared to L4D where you'd get just over 1000 on average. The Special Infected were also far more common, think I got 18 smokers MYSELF, not counting the other 3 guys. We even failed one of the levels, #3 I think, our first try...on NORMAL difficulty, and we were all experienced L4D players to varying degrees.
All in all, considering I spent $45 on this (and only $25 for L4D on a weekend sale) I think I'll get my $$ worth! I had over 110 hours in L4D, I'll easily pass that here.