Better campaign and graphics over the first game. Versus mode, not so much.
In L4D2, the first thing that you recognize is the better graphics. I only have a Nvidia 9500 GT graphics card yet I can still run it at good settings. The game also adds in different environmental situations unlike the first game which was very limited. (Eg. Day/night, fog, rain)
Next, L4D2 does not string along the same characters from the first game. They introduce four new characters with new personalities and funny lines. You get a brand new storyline with 5 connecting campaigns.
L4D2 also adds three new special infected. The difficulty on this game is higher than the first due to the more advanced AI director which is a lot better at coordinating special infected attacks that can really destroy your team if they catch you off guard.
The only complaint I have against the game is that I feel as if versus mode is a lot more difficult than in the first game, but it is more realistic in the process.
And, of course, your opinion on shooting thousands of zombies for hours on end is the real decider on whether you want to get this game.