I was an early adopter of the game, and much like TF2, I'd gotten myself burned out on it.
I had purchased both with the understanding that Valve would release sufficient content drops to make the purchases worthwhile. In TF2, that was certainly the case.
Left4Dead, not so much. The game consisted of approx. 10 multiplayer maps, and 20 cooperative & single player maps. That sounds like a lot, but, in multiplayer, a full 'campaign' takes about an hour or so to play. Further, because the campaign is pretty linear, it didn't take long for experienced players to quickly realize what you should and shouldn't do, and where you should and shouldn't go as both Survivor and Undead.
Ultimately, it got repetitive. Not to say that wiping out the survivors didn't get less sweet, rather, it got more so - you knew the strategy for survival, it was always a question of breaking the survivors out of their plan.
So, two things happened:
1) I started playing the game again with former co-workers (to which I'm called 'The Lucky One') and it is a hoot when you are playing the game with people you know. The enjoyment factor is much much higher.
2) The release date for the new content was announced, which caused Point 1.
So, I've been enjoying Left4Dead again. Now, if they finally get the SDK released, like they said they would, the possibilities for awesomeness will only increase.
1 week ago