Yes as blackhawk said.............Left 4 Dead 2 is much improve, its basically everything or almost everything they wanted to put into Left 4 Dead but didn't manage to. Thats why they came out with the 2nd one so quickly after the first one. Its well worth it, i just ordered it for $30 which was an amazing deal during black friday sale, awaiting its shipment arrival.
The new one has new maps, modes, weapons, melee weapons, new zombies, more gorey graphics, new special zombies, new characters and basically almost new everything. They include all the old weapons but they seem slightly changed/revised and the new weapons are sick, deagle is epic and the AK and SCAR are really cool also. The Katana and machete are sick and its awesome slicing up some zombies.
Yes, download the demo and play it, you can play the demo online but only one campaign and it gives you a really good idea of all the new features in the game but i don't think it includes everything, it includes almost everything though.
The first left 4 dead was great and the 2nd left 4 dead is amazing also.