Even when playing as a boss monster, co-operative play is where it's at. Alone, you're susceptible to easy counter-assaults from the Survivor team. A Hunter is no match for even two people together. It needs a Smoker to grab someone, and a Boomer to puke up a swarm-summoning load.
Versus mode is the way to play this game. Singleplayer is fun, and co-op is great, but Versus mode demands you take note of how far the other team made it into the level, creating an impetus for your own run through. You accept death, if you manage to get just that little bit further. Even though a team that sticks together wins, it's worth powering though a level simply to get as far past the other team as possible. Which is just funny as hell when you manage it.
Time to pick at the loose threads. Although it makes conceptual sense that your weapon choice is slim, L4D could use a bit more choice for the survivors. Also, it occasionally frustrates when things don't go your way in Versus mode: the random apportioning of the boss classes when you play as the infected can get annoying, particularly if you're forced to play as one class you're not very good at, or you don't get to play as the Tank.
If you're not going to play at least in co-op you'll miss out on much. The AI is good, but it does feel a bit like you're leading ducklings into battle. They really can't replace the human element. But then the '4' is right there in the title, folks. Get more friends. Only the four scenarios I've mentioned will be available at launch, and of those only the hospital and farmland will be playable in Versus mode, although needless to say, Valve will be filling in the gaps with subsequent Steam updates.
But that's not the point. You'll go back and play the same map again and again anyway, because no two run-throughs of Left 4 Dead are the same. The few static elements are just there to anchor you; the AI never does the same thing in the same place. I've opened a door to a house to find a Boomer behind it, waiting. He wasn't there the second, third or fourth time I played. By turning the spawns into a dynamic event not tied to the level architecture, Valve have made an endlessly replayable game. It's got all the nail-biting high tension of a control point assault/defence of Team-Fortress 2, mixed with the tightly designed levels of Half-Life 2 and the weapons of Counter-Strike. As long as they keep introducing new missions, there's no reason why it won't be sucking on your bandwidth for years to come.