It's not that different, it's mainly just some rebalancing and changing the sound effects. For instance, the pistols in L4D2 feel weaker because they have less flash and quieter effects, but they actually do more damage than they did in L4D1. Also, the shotguns became far less uber by reducing the number of infected they could penetrate (instead of having infinite penetration like the rifle).I recall thinking when i started playing L4D2 that it feels different when you shoot and other small stuff like that.
The other things you mentioned are simply map design flaws and gaps in special infected gameplay that they fixed by adding the new ones etc. Nothing they couldn't fix with the previous game, in an expansion or a big content patch...
I really do wonder why they went with a whole new game instead...
Honestly though, I think the biggest problem people had was that they equated "continued support of the game" to mean "We're going to treat this like TF2 and give you shitloads of free stuff for years."