Left 4 Dead 
Halo 3 Overall Comparison I've always enjoyed a good team game more than a single killing romp. Left 4 Dead is the saviour of online gaming for the 360. As more and more people get Xbox live, less people seem to talk and work as a team.
In most games, it's not much of a problem, unless you get stuck on the losing team, and no one will listen to your comments. In contrast, if you decide to go for a single player romp in Left 4 Dead, your screwed.
Its taken people awhile to work this out, and, granted, there are still alot of young kiddies grabbing the Xbox controller and trying in vain to run through a level by themselves, but alot of games that you get pulled into have some teamwork in one way or another.
Sure, Halo 3 was a great game, but no one talks on it anymore, and when they do, its insulting geek speak, and it becomes quite boring quite quickly when its like this - there is not much of a communicative community within the games.
On Left 4 Dead, its just far more fun, even though theres only a few levels, the game is just so hilarious, and brings people together over Live. Its a game that will never get old for me, whereas, Halo 3 did fast.
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Comments
I definitely play L4D more often; it's a very good co-op shooter. But on Xbox 360, Versus mode just doesn't work.Halo 3 has an all-around great experience to offer, with Campaign (yes, it isn't as good as the other two, but you can play it with 3 friends!), Multiplayer (I still don't understand what people don't like about it other than lag...), Forge, and Saved Films.
The only reason I gave this a thumbs-down is because this comparison was even made. Halo 3 also died down quite a bit for me. But in my eyes, they're still equal.
(But I still don't think Halo 3 is generic. If anything, COD4 is generic.)