Hey, don't mean to spam up this thread either but I wanted to weigh in on my thoughts real fast.
The Halo series are some of the best selling video games of all time. My sister works at Game Stop and her midnight release of Halo Reach brought the store 26,000 dollars. The only game that may compete with that in the near future is Call of Duty: Black Ops and maybe, maybe Medal of Honor.
Halo revolutionized the FPS industry in that it gave users a chance to play with sick weapons, sport neat armor, and kill nasty bad guys, and moreover, do it all online. Never mind the poor story line, most people I know playing the game, are playing it on Xbox Live.
As far as Left 4 Dead goes, its an Ok game, but no where near as influential as Halo for obvious reasons. Though cooperation is fun in this game, other games do utilize a great deal of cooperation though maybe not to L4D's extent. Games like Gears of War for instance, feature cooperative campaign modes where survival is dependent upon each other. Gears also features a more intense, and more interesting online multiplayer mode. Also, L4D initially only gave users the chance to wield, what, 4 weapons? A shot gun, assault rifle, sniper rifle, and an uzi or some such. Pistol too? Not very dynamic and rather disappointing, IMO.
That being said, I've played both Halo and Left 4 Dead and I prefer L4D for the campaign and the sheer excitement. But I'd have to hand it to Halo for its innovation and online gaming.
These are of course, my opinions. Either way, I purchased a PS3 to compliment my Xbox. This way I can play online for free. I play Call of Duty and that's really about it.
My two cents. Carry on!