Montegue seems to be getting a lot of hate for this but I can totally see where he's coming from. I'l probably wait for it to be on a half price weekend and get it in a few months time.
I can't put my finger on it but it definately feels like something's missing. I've just finished the demo and my first impressions were:
There's something up with mouse aim, I don't know what but it doesn't feel right. As soon as I started the game I set it to how I have all my other source games but it still feels odd. It was either overly smooth and floaty (regardless of mouse smoothing being on or off) or jagged as if the crosshair can only move along a grid pattern.
The textures seem very flat. I know l4d was hardly a graphical powerhouse but it seems they've taken cues from tf2; which contrasts badly with the new improved gore. Gritty guts and dimemberment covering the floor, yet the sleeve of my character looks like I'm using the engineer.
On the topic of gore, I think they did a good job on it. Pipe bombs are MUCH more satisfying to use now.
I'm unsure of the new weapons, some are good, melee seems overpowered. The magnum/deagle thing was really good, nice and powerful and very accurate. Noises seem better too. Weapons seem to take up a lot of screen real estate though.
Encounters with boss infected seem a lot more common, but they seem easier to kill - that could just be more powerful weapons though.
Zombie path finding actually seems worse than in l4d. I had a horde come after me in an open street, yet I still get half of the zombies trying to climb over an unobtrosive streetlight rather than just run at me. There also seem to be a hell of a lot of "floor gliding" zombies too. I also encountered 2 hunters during my playthrough, both attacking scenery rather than me.
I feel the more open ended levels work, there was never a clear indication of where to go, but I never got lost - it just worked.