Having played only three days of L4D and one day of Versus I managed to do some surprisingly slick moves with the Special Infected, though I'm quite happy I didn't end up as the Tank as I would have failed spectacularly.
Did you know you can wall-jump with Hunters?
It's really fun.
Boomer is the most fun for me though.
Hunters can easily be either killers or distractions. They move quickly and can jump off walls. I love jumping wall to wall down a hallway, confusing the hell out of my opponents, only to land in the middle of them as my teammates descend on them.
Boomers are great, adding damage, confusion, and zombies to any battle. A good boomer will get 3+ survivors goo'd every time. Anything less is almost a waste.
My favorite is playing as the tank. =P I have only twice failed to wipe the entire survivor team as tank.
Protip: Once you learn the patterns of the tank, you can actually run AT him and slide to the side before he swings. I've danced around tanks for minutes on end and killed them solo with an auto-shotty, both against an NPC tank and a PC tank. It isn't easy though, and I'm usually on stimulants. =P The cinematic at the beginning is almost identical to the game-play no matter your level of play.
Protip #2: Do not be afraid to melee as a hunter, or any zombie for that matter. If you have no other alternative, melee is not the worst choice as it does, at the very least, 1 damage, but I've heard it's much more than that for the PC zombies.
Example: You tackle a PC as a hunter, doing 3 damage (It tells you how much damage you do with your tackle, and damaging tackles is a relatively new aspect to the game. Before it did no damage) then you get knocked off by another PC, only to melee slash three more times... equaling or surpassing your tackle damage.
Protip #3: As a boomer, try to keep within range of their guns and out of range of their melee. Attempt to blend in with incoming horde so that as they shoot the horde, they shoot you, exploding all over them. Expect a delay and short range from your vomit and plan accordingly (take them by surprise).
Protip #4: You can spawn within feet of the survivors as long as they cannot see you and you aren't within the prohibited range... that means ducking behind a car or standing behind a slim tree means you can spawn right next to the survivors and puke all over them, etc.
I wish my PC could play this game. As it is, my friend and I spent many, many hours on his top of the line PC, eventually getting the Zombie Genocidist achievement in less than a month (53,000 some-odd zombie kills).