Graphics seems a little better than Left for dead 1 but ran just as well for me on an aging mid range machine.
The maps feel bigger and less linear but it is hard to draw conclusions based on only two maps in the demo.
The infected look scarier.
The new types of infected add more variety to the game but I suspect there isn't enough in the demo to fully appreciate all their roles.
There is a wider selection of weapons which is good. There isn't such a simplistic distinction between normal weapons and upgraded weapons even though some weapons are definitely more powerful than others. My favourite so far is the AK47. It is very powerful and insanely accurate.
The melee weapons are fun but gimmicky. If you equip a melee weapon you lose your pistols and twin pistols are just better than a melee weapon. Nevertheless I imagine there will be lots of fun to be had with melee only servers and such.
There is a wider range of accessories available including adrenaline and a defibrillator. In our run throughs on normal difficulty we didn't have to use any of these. Perhaps they play a bigger role at higher difficulty settings.
I really like two parts of the demo - a park where you thread your way though hedgerows avoiding or confronting infected as you see fit and also the "alarm" section where you must run a twisting path through a gauntlet of infected to silence a ringing alarm. Sadly the demo peters out after this gauntlet run - it ends on whimper rather than a bang.
Overall impression - good but not spectacularly better than L4D1. I do wonder if the demo does the game full justice. I think a lot of thought went into the new special infected and the new weapons but there isn't enough space in the demo to see them all in their intended roles. The spitter for example spews toxic bile on the ground which is a minor nuisance in the open maps of the demo but could be deadly in a narrow indoors map.