
Despite my initial fears, L4D is still great, and has been eating most of my free moments over the past few months.
The game is survival horror, and you choose one of 4 characters to survive against entertainingly large amounts of zombies. Mixed in there are 4 (also playable) special infected types. You generally play with other people, and usually against other people.
After prepurchasing the game, I was certain the full version would have more variation in it than the demo. More infected types, more survivors, more weapons, more levels. To my dismay, it only had more levels, but I keep playing, and it's still fun.
Fortunately Valve continue to support their games post release, so this game will probably continue to evolve over time, (just as Team Fortress 2 has) maintaining a strong userbase in the meantime.