Not really bad, but not really great either!
The lack of a greater variety of weapons and opponents in L4D . Within the 2 hours of game-play a person experiences pretty much all this game has to offer.
Graphics are pretty good considering this is an older engine. The splatter effect is done really well.
The learning curve is probably within the 1 minute range and is by no means steep.
Sound is mediocre, but could be improved regarding weaponry. Sometimes the characters catchphrases are downright hilarious when repeatedly used in multiplayer.
The AI is better than most titles regarding spawning and behaviour and is a nice change instead of the usual predictable behaviour that many titles suffer from. The "Horde" idea is definitely original. The main gripe about the AI is the way some characters can haul you down to the ground with the only way of escape is relying on a team-mate to save you…. very cheesy. However, the way teammates can help each other health wise is a good idea and could be seen as a countenance for the cheesiness.
Multiplayer is obviously the strongest point of L4D and probably the most fun considering you actually get a team that cooperates with one another. HINT: At least play the single player in order to train yourself as patience is not a virtue within most servers. Unfortunately, Valve has also left out the ping rating for servers so finding a good one may prove challenging.
Replay value is somewhat affected by the lack of weapon variety, AI opponents and the length of the game, but doesn't kill it. Multiplayer is L4D's greatest strength for variety of replay, but is also limited by the lack thereof. Of course there are four additional single-player levels after the player completes the first one. But ends up being the phrase, "Wash, Rinse & Repeat".
Overall, I believe L4D had greater potential than what was offered to the public. No fear factor was incorporated within considering this is an action horror genre. The single player component is way too short for my taste, which is why the reviews rave about the multiplayer being the best. A good title should be strong in all it has to offer and not just one area or relying on the community, i.e.: mods, skins, etc, to make it better.
Don't get me wrong, L4D isn't as bad as some reviewers say it is and it does deserve a better rating than, at least, 5. However, this title is far from being perfect and doesn't deserve a 10 either. Of course we all have our preferences so I'll leave you with this: The best you can do is either rent it first or wait until it falls into the bargain bin category. L4D is worth a purchase, but not an expensive one!
Nuff Said!