Not overly impressed so far... Melee weapons seem pointless other than to add an option and dilute gameplay for the sake of it.
Likewise with the numerous weapons. Many seem to really just be variations of another in different guises.
My favourite thing so far is the swat zombie. This actually brings in a variation to gameplay, and it was something I proposed on the Valve forums ages ago. Although I suggest only a head shot would work. I also suggested some of the infected if you shot off their legs they'd crawl towards you. Again to add variety...
I guess it's only a demo, so we won't know until we see the full thing. A NPC model and zombie dogs have been spotted in the game data!?