I was playing the L4D2 demo last night for about 45 minutes. A few observations:

1) This probably does not deserve to be labeled a "2", I don't think it's that great an update. Yes, it's an improvement and yes, they added quite a bit of content, new scenarios, etc. - but it's still L4D.

2) I LOVE L4D in general, so I would pay the $45 anyways for the new content

3) It seems the graphics and details are 'smoother' and even seem to run a bit better on my machine.

4) Although only 2 maps in the demo that I played, The games did seem to 'flow' very well and I'm enjoying the new melee weapons alot. However, in order to use a melee weapon (Bat, crowbar, night stick, frying pan, etc) you do give up your pistol.

5) I think the new infected are a nice addition to the game and the new weapons are pretty cool which adds a new dynamic to the game play. I like the "slime" on the ground and if you stand in it, it hits your health.