I've been partially enjoyed Left 4 Dead since its release on the (non-zombie) unsuspecting public. The problem i have (and possibly with other Valve games like Team Fortress) is that, despite coding into Steam specific Groups that you can belong to, you can only join games with the general public or with your friends.
This means that if you have a rather large group you would have to friend-up every single one of them to be able to play with them. This is stupid - to say the least. Why have the group functionality of Steam if you're not going to be able to use it in Steam games? Why do i have to add 700+ people to my Steam friend list if i want to play with them in Left 4 Dead rather than just being able to join a game with people from the group(s) i'm in..... it would be so much easier and i think it's a big oversight from Valve. I really hope they add this functionality in otherwise i'm going to end up hardly playing L4D.
Why? Mainly because i want to play with my 'friends and acquaintances' but also because the co-op AI really sucks on any level that's higher than normal. They have no sense. Valve hasn't programmed them with 'thinking of the player' in any sense.
For example: In the many set-pieces within the game where you must hold a position they do not act accordingly - they do not position themselves in useful locations allowing the player to effectively hold said position against the zombie hordes - they will frequently run off together while you're manning the minigun, leaving you open to attack from behind and they will frequently remain out in the open rather than being near the player and being able to cover and be covered by the player. They also frequently drop down off of a high point you may be holding - letting themselves get overwhelmed by the zombie hordes.
Secondly, they are really stupid when it comes to general gameplay. I was jumped on by a hunter in a recent game and, while the AI stood there, right next to me it decided that instead of meleeing the hunter off of me that they would instead reload and proceed to shoot it off.... so i lost a lot of health right there and then. Then there's the whole gift-giving aspect. If i give a pack of pills to an AI i want them to keep them.... not give them back to me instantly or to another AI!!! I gave them to that character for a reason!! I really want a simple, 'come here' 'go there' command like was in Half Life 2 for the squad AI so i can easily direct them to picking up bonuses like Health packs and pills or ammo etc. when other times they'll wander off because the AI all-of-a-sudden has a psychic premonition as to where in the level these bonuses are... though they often neglect to play the sound bite file to let me know that they found some stuff.
A few tweaks and the game would be almost perfect..... To be honest, if i didn't care about the game these issues would be non-existant.