Everything a Sequel Should Be
Let's make it pretty clear here, the core of the gameplay remains in tact, though much has changed outside of that.
New special infected: The Spitter and Charger work great for countering corner-hogs from the first game, and the Jockey just helps in almost every scenario, as long as he's coupled with another infected.
New Levels: So far, so good. Level design is great, and even has more character to it. Little things like parachuter's being strung up in trees, or chargers being tied up like a sacrifice. A few of the water levels get slow, which i understand is trying to introduce another type of panic but not my cup of tea.
New Guns: All the guns work great. Feels like there are more than just the 7 guns littered throughout the world now, and the new ones feel even better to shoot. Laser scopes do little more than your aiming reticle, however the incendiary and explosive ammo is great, and the grenade launcher has its own fun to it. Boomer bile i find doesn't work as great as it should, against enemies, but works well enough to get rid of a horde, much like a pipe bomb.
Melee: Crazy fun, and very useful. On any game mode they have their place as charger killers, hunter killers, jockey killers, and for blocking off doorways for instance. Incredible fun to chainsaw or katana through zombies.
New Game modes: The original game modes stay in tact, with some scoring improvements to versus, survival being kept the same(however i may be wrong but seems like you can only do finale's :() and campaign being the same, although different levels of course. Scavenger is essentially the best multiplayer mode, as it's a short version of versus, and more competitive in most cases. Gathering gas cans from around an area, with faster spawns, and more ways to hinder the survivors(spit the gas cans, hunter onto a gas can in hopes that teammates shoot it). In general a great mode. The realistic mode is essentially campaign except without a few things that help like outlines, and more difficult to kill zombies. This is fine, only for the diehards really, which is nice but not an entirely new mode in itself.
Essentially, it's the first game, but a lot better. The moving crescendo's in campaigns really change the gameplay up and make things more intense. If you played expert in the last game, you won't necessarily be able to in this game. The moving crescendo's, new special infected, often double tanks, and uncommon common make it harder to beat levels.
After the game's release, valve has supported the game through "mutation", which changes the gamemode and tweaks in gameplay every week. This keeps the experience somewhat fresh.
Overall, fantastic game, if you enjoyed L4D even a little, make sure you pick this one up. If you're unsure, wait for a weekend deal. It's got significant replay value, and with friends there's few games that beat it.