Great improvements in level design, but not the leap forward the series needs.
The graphics in this game are an improvement on the first game but not by much. For me the source engine is one of the best as it looks decent on even poor systems and in this game you will see some improvements, especially flame and smoke effects are much improved in this sequel.
You also have a greater choice of weapons in this game, with several weapons offering a choice of playstyles, meaning you won't just see everyone carrying the auto-shotgun as in L4D. Having said that the much advertised melee weapons are virtually useless on any setting other than easy campaign, given that they are no use in versus matches and in realistic campaign modes, due to their lack of range and effectiveness against special infected.
The major improvement for me in this game is the level design. Gone are the boring, dragging levels of the first game and replacing them are exciting story driven campaigns. Firstly the campaigns here are contiguous, leading to a far greater sense of cohesion. Secondly they are very well designed and keep you moving and fighting rather than encouraging you to camp in one room. Imaginative objectives are inserted in order to justify your play as well as situations that are simply awesome to play. For example one level in particular has you fighting your way to a gas station to get fuel and then returning to your boat, only by now a storm has risen and you must wade through the water to get back, with the wind and rain cutting down you visibility.
The game in general seems to be more difficult as even on easier levels you will be attacked by multiple special infected at one time and a particular improvement for me is the realism function, where most elements of your hud are removed, meaning you can no longer rely on the outlines of your comrades to find them and thus feel greater immersion into the game.
However several problems with the game are still evident particularly in the server program. Although there are improvements such as the game automatically trying connections to any server type you have specified, rather than one specific game, you will still regularly face the scenario where a group of 8 people will form a lobby for a versus game and connect only for 3 or 4 of the players to be dropped by the server when you finally load the map. Also, due to the problem of people not connecting to individual dedicated servers there are often not any plugins to prevent serial offenders such as teamkillers joining the server you are on.
For me though, this game is left 4 dead as it should have been. Exciting levels, greater weapon choice and better gameplay. The problem is that playing the game feels just like playing left 4 dead and from Valve I expect more.