This game is awesome - it paints a great picture of a zombie apocalypse and has many "Oh poop!" moments.
Left 4 Dead has been called
the anti-Counter-Strike and I think that's pretty accurate. Counter-Strike was meant to be a team game but solo/cowboy types can easily do well in the game if they have adequate skills. With L4D no matter how good you are you
need the help of your teammates. It makes for some very fun times and it's even better with some buddies (and maybe even a couple brews - it's not drinking alone if you're playing a game online with friends).

Beer not required but helps.
This is no way a comprehensive guide and I'm sure there are better strats - this is what I've found in my limited playtime with friends. If any of you want to throw down join the
TrueNuff Steam Group and we'll take on the horde together.
Teamplay
The Budday System
Stay cohesive and travel in in a group or at the least in pairs. If one of you get snagged or toppled the other can help.
You Make a Better Target Than A Window
Ducking improves accuracy and lets your teammates shoot over you giving you a nice stepladder effect - like a shrubbery.
Don't Cross the Streams!
Team mates might shoot you from time to time but it's a two way street. Try to not run in front of teammates while they're shooting and be aware of the lines of fire. This is especially important on expert where a shotgun blast can incapacitate your friends.
Search Rooms Once, not Twice or Fourice
When you fan out to search rooms don't cover the same ground. Tell your teammates what you find (weapons, explosives, pills or packs) and tell them how many so everyone doesn't waste time and have the special infected creep up and snag one of you.
The Infected
Regular Everday Normal Guy Zombies
Use head shots as much as you can for common infected - it's more economical (takes less bullets).
Shoot The Tounge
If you're in a pinch, can't see the smoker but see your teammate is snagged just shoot the tongue. Sure the smoker will just come back - but he respawns if you kill him anyway.
Hunter Punter
That achievement exists for a reason! You can shove & stun the hunter while it's flying through the air. Try moving backwards so he falls short of his jump then smack him. It's pretty satisfying to take him out using only shoves too.
Pipebomb Boomer Diversion
Pipebombs are great for Horde or Boomer attacks since they distract most of the zombies and take the heat off of the team. Announce when you use them so you and the team don't use two at the same time and waste them.
Which Witch?
We usually just avoid the witch - it's not worth it and there's nothing to be gained. Someone usually just dies or gets badly hurt. Just run by her as fast and far as you can. If you
must deal with her have someone trigger her from a distance so she chases them and then the other team members can take her out before she gets to them.
Fight Fire Tanks With Fire
Save mollies (molotovs) for Tanks. When you spot a tank, light it up and run around until it burns to death. Ladders are good because they buy you time. Run up a ladder, have the tank chase, you jump down and repeat.
Gameplay Tactics
Heal When Grey
While it may be tempting to heal when you're around 10 health, you'll want to save your packs at the higher level and heal when your vision is grey (one more hit and you'll die). If you have 10 health, get knocked down and picked up you'll usually have 30 health which will bleed down. You can't bleed to death, the only way to die is to be overwhelmed by zombies so if you play as a team you'll be fine. Pop pills to give you the temporary speed boost if you need it.
Stop, Drop & Shove!
When stuff gets hairy, like a horde trigger or boomer attack get everyone to pile into a corner (or narrow hallway or wall if you're in a pinch) and just keep shoving. I didn't think shoving did much when I first played the game but it's powerful as hell. A good shove pushes 3-5 zombies at once and can stun special infected attacks.
Run & Choke
When my friends & I first played we tried waiting and taking our time and that got us killed. We also tried all out rushing and that's alright until you hit a tank or horde event. The strat we use most these days is a combo of both. We rush as hard as we can but when stuff goes down we Stop, Drop & Shove in a corner or hallway (choke point) until it's clear and move forward. After the tank event you can usually rush a bit harder without worry unless you get double tanked - that sucks zombie nuts.
That's all for now - BOOMER!!!
Awesome game awesome blog! :)
Thanks Alpha :)