I'd like it to be noted that I also own L4D on the 360 and have played survival mode on the 360.  As with most FPSs, I enjoy it more on the PC.  Furthermore, I find that people are more cooperative and less idiotic on the PC.

As of last night, I have earned 3 gold medals, about 5 silver medals and 8 bronze medals.  The gold count is definitely accurate, give or take one on the bronze and silver.  What do these medals mean?  Well, my friends, a few things:

1. The fact that you ask this question means you haven't played Left 4 Dead.  This is a serious and grave mistake.  Rectify this as soon as possible.
2. If you are unable to earn any medals at all, you either have horrible luck in finding a group or you quite simply should stop playing (alternative, practice practice practice!)
3. The medal indicates how long you were able to last on a given survival mode map.  Bronze for 4 minutes, silver for 7, and gold for 10.
4. How "easy" it is to achieve gold medals is an indication of how much better you are than Valve engineers ;) (these times were based on their own performance)

What a survival mode map?

   It's a slightly modified map portion from one of the fourth campaigns in which Infected are thrown at you until you die, basically.  You get an unusual amount of tanks, smokers and hunters thrown at you.  Boomers are more irritating than anything else and don't, in my opinion, cause a significant problem.  There is one brand new map that isn't played in Campaign or Versus mode and that is the Last Stand, the Lighthouse map.  This map is actually quite difficult, I've found.  It's a very open area and the amount of explosives they provide you with initially is an indication of how much firepower you need to actually survive.

   The existing campaign maps are slightly modified in that certain areas are blocked off.  You have no actual chance of winning or outsurviving the flow of zombies since they will keep coming until you die.  There have been many instances of 3 smokers standing beside eachother and snagging 3 survivors at the same time.  This can be be particularly disastrous, especially when a tank is already rampaging around.  Similar thing with 3 hunters. 

Are the Infected cheating?

  I'm sure this isn't actually a question most people have at first, but the more you play, the more you notice the liberties Infected seem to have with certain maps.  Issues I have noticed include:

• Smoker tongues going through hard surfaces (wood surfaces, metal pillars, around corners, through the edge of concrete).  I suppose this could make sense if they can weave around things, but the visual clipping makes it frustrating.
• Hunters resistant to punting.  Often when meleed, they don't get knocked back.  So more often than not, if a hunter wants to pounce you, you're going down.
• Boomer vomit going further than usual.
• When playing a Hunter in versus, if you put yourself on fire and then pounce on someone, once you are killed or punted off the survivor, the survivor is not on fire nor are there remnants of fire around them.  In survival mode, the fire seems to actually be transfered to the survivor and you remain on fire after the hunter is killed.  This may have changed in Versus mode in the last little bit, if so my apologies.

  As much as I'd like to complain that tanks are cheating, they are understandably extremely strong and difficult to kill.  I haven't noticed any abnormalities as of yet, though there are times that I scream "BS!"  I do recognize that most of the time that's just sheer frustration.

What makes survival mode so great?

  I have previously (in another article) highlighted that Left 4 Dead is an amazing coop game.  And I still stand by that.  I would actually argue that this mode has emphasized that coop element to a degree not quite achieved by other games.  To put it simply, if your group is subpar, your time achieved will be embarassing.  Cooperation is essential to survival, so if your group style doesn't jive with any other group members, you might achieve a mediocre score, but I doubt you will achieve a gold medal.  Communication is also greatly emphasized here.  While it is possible to do well with only a couple members using mics, this seems to only be true on the "easier" maps.  Dead Air's The Terminal and Blood Harvest's The Warehouse primarily come to mind.

  The big trick is learning the maps, and I find this a lot of fun.  I enjoy trying the different positions and techniques, as well as trying to see which combination of guns work best.  You should always have at least 2 people with shotguns, in my opinion.  You need the extra punch for tanks.  If someone is efficient with it, a hunting rifle is ideal to deal with long range smokers and even to take down any hunters that are just beginning to creep over.  Without a doubt, unless you're standing right on top of the ammo pile, you should use your pistols as much as possible.  Only tanks really need to have special ammo used on them, otherwise you can technically make do with the pistols.

  Healing becomes an issue, obviously.  You only have a total of 8 medipacks, but quite a few pills.  In my opinion, it's best to only heal when absolutely necessary (or if you're nearing the 10 minute mark, because post 10 minutes it gets absolutely hideous out there) and rely on pills.  Don't be afraid to share your supplies if needed - this is a coop game, not last man standing wins all.  Pipe bombs should be utilized to give the group a much needed break - to heal, to resupply, etc.   Molotovs, on the other hand, should be used against large waves of zombies and against tanks. 

  There is no running away in survival mode.  Stand tall.  Or crouch and shoot like hell.