I’ve gathered data in-game, scoured forums, news releases, and blog posts about the new unlock system now in place since the latest Team Fortress 2 Sniper / Spy update patch. From this I’ve established how the new unlockable system works, with some interesting results and discrepancies.

TF2 Unlockable System FAQ:

FINAL UPDATE 6/1/09:

• Since the new drop system’s release Valve has made a couple of small updates and players have gathered data to provide a clearer view of the drop system.
• According to data gathered by players, the drop system runs off an interval of 25 minutes. These intervals are unique to each player.
• Unless the bug has been fixed, players MUST be connected to Steam Friends in order to receive item drops. If you have played for many hours without a single drop, you are likely not connected. This seems to be related to the Steam Cloud’s system, but should be patched soon.
• Valve has reintroduced milestone achievements. (If you complete enough achievements, the new Sniper and Spy items will be unlocked as was the case in previous class updates). Also it’s worthy to note that this update brings with it the easiest milestone achievements yet. Within a few days of play you should have no problem unlocking all 3 items.
• The system still appears to give you 1 unlock about every 2.5 hours or so…on average
• Hats have been calculated by players to drop in the range of 0.5% of each time you successfully receive an unlock.
• Valve will soon release the trading system now that the random drop system appears to be operating correctly.
  

Important update–troubleshooting for players with 0 item finds.

• The below drop frequency stat I calculated appears to still be correct (you should roughly have a 35% chance to find an item per hour)
• Valve may very well have recently updated the system (Monday 5/25, around 3 PM CST). They did verify that it has not been working as designed all weekend. (Finding drops should be a regular occurence according to the dev team.)
• If you are one of the numerous players who have clocked many many hours, but have not received a single unlock, make sure to check your old Medic, Scout, Pyro, and Heavy achievements. Make sure that the number of achievements you have earned matches up with your currently achieved Milestone. I have personally had experience with this. Up until noon Monday I had clocked about 20 hours (10 hours play time, 10 hours idle) with absolutely 0 items found. A few minutes ago, I checked my achievements and found I had the Medic and Scout 3rd Milestone, but did not actually have 22 completed achievements. I fixed this with the Steam Achievement Manager 5.0 (use at your own risk, but seems safe. Do not run this while connected to a VAC server.) then rejoined an idle server. Within 5 minutes I had found my first unlockable item–the Ambassador, then 30 minutes later my second, the Huntsman. 20 hours vs. 5 minutes with this fix has me convinced.
  Summary: if your achievement milestones are not correct, this may be the cause of 0 drops.
• Many players are also reporting that you may need to be connected to the Steam Friends feature. If you have been experiencing 0 drops, be sure to go to Steam > Friends > Sign-In. This appears to confirmed widespread, and after I connected to Friends (and did the above milestone fix) my 0 drop problem was fixed.
  

• Can I be in spectator mode and still find unlockable item / weapon drops?

  Yes. You do not need to be playing, and you are allowed to be in spectator mode. There are no hard stats on whether or not you may have a decreased chance of the random drops, but it is confirmed that players have found the items while in spectator mode and completely idle.

• Can I find unlockable weapons while idle / AFK?

  Yes, but it appears you must at least choose a team (red/blue/spectator) to fully enter the server.

• What is my percent % chance of finding an unlockable item?

  According to the information the Team Fortress 2 team has released, the system should be set up so on average, players should find about 1.5 items an hour. It does not appear to currently be operating as advertised (keep reading for more information).

  This 1.5 weapons an hour is based off the information Valve / TF2 released that over 2 weeks time, players should find 20 unlocks–and that this is based off the play time of an average person. Although I was unable to find hard statistics on the average TF2 playtime per person, I estimated 6 hours/week would be close.  (20 drops divided by 12 hours playtime = 1.67 or about 1.5 drops an hour rounded.)

  Now, according to data I have gathered in game, the above 1.5 drops an hour is NOT a reality. By my calculations, each player actually has roughly a 35% chance to find an item every hour.

  This is based off the following: in a full server of 32 players over my hours of playing in the past week, I saw that the random check to determine if players will successfully find a weapon or item drop took place every 3 minutes (roughly). At each of these 3 minutes I observed up to 2 players finding an item each check. Based off rough data of how often 0, 1, or 2 players found an item during a check, I determined the odds that an individual player in the server would find a drop was about 2% at each of these 3 minute interval checks.

• How does the unlockable weapon random check / time interval work?

  It appears the time interval for the random check system is controlled by the server and runs about every 3 minutes. Each time the interval is up, a random check is run for each player in the server all at the same time. This is based off the information that I have confirmed multiple players finding an item at the exact same time (albeit on rare occasions). Based on the above calculated 2% chance for each player to successfully obtain a new item or weapon at each 3 minute interval, this makes sense.

• Will I find items if I create a server to idle in? How do I set up a server correctly to idle in?

• Yes, you can create your own server, even by simply using the “Create Server” link in the game’s main menu. Reportedly you may not find items this way however. To be safe, either join a server running the achievements_idle.bsp map or do the following to customize your personal server settings:
  • Choose Create server making sure to set the number of players before you hit start then type this in console without quotes:
  • “ip x.x.x.x:27015” (after you have forwarded the port on your router.)
  • “sv_lan 0”
  • “sv_pure 2”
  • “password yourpassword” (don’t use this if you want people to join)
  • “mp_idlemaxtime -1” (now noone will ever be kicked for being afk)
  • “changelevel Name_of_level” (use any level you want to.)
    
  • After the game changes levels check the console to make sure it says that you are connected to Steam and that VAC is enabled. Also doublecheck your item inventory screen to make sure your custom weapons screen will open up.
    

• Is there any way to find a specific new weapon, or to find drops for only a certain class?

  No. Currently the drops are completely random and players can do nothing to change this. According to the Team Fortress 2 dev team, this should change soon.

• How can I find hats or items to put in the headgear slot?

  The only answer is to play for many hours. The hats are the first of many future rare items to be implemented and the drop system for them works just like all the other items–completely random, with better odds of success over greater time periods. Finding this hats as opposed to any of the other unlockable weapons is much much less likely.

• What does headgear do?

  Hats and other types of headgear appear to be for looks only. They do not affect your gameplay in any way at the moment (other than to flood the chatbox with questions about your crazy new hat).

• Is there any way to cheat or manipulate the random drop system to find items faster, increase your odds, or find a specific item?

  No. Though many users have been joking or starting rumors about using admin priveleges on a server to grant players items, this is not true. The only advice I can give you is join a server that does not kick idle players and let the game sit overnight in spectator mode. Look for the map achievement_idle and find something else to do.

• What are the new unlockable weapons?

  For the Sniper: Razorback (slows you but prevents one backstab then breaks), Jarate (throw for splash effect-puts out allies on fire, causes enemies to take +35% damage for a time), the Huntsman (sniper rifle replacement-better at medium range).
  For the Spy: Cloak and Dagger (allows spy to recharge cloak, while cloaked, but only while standing still), Dead Ringer (scout fakes death and recloaks upon hit), the Ambassador (revolver replacement, increased accuracy for 1st shot, reduced damage compared to the revolver).

My own opinion on the new system, active since the Sniper and Spy update went live on 5/21, is mixed but mostly negative. Admittedly this is largely because I have been unlucky enough to not find a single old or new unlockable weapon in about 8 hours of play. I understand not tying the unlockable system to achievements, however if the system is not based in some way on performance I see no reason not to simply give all players all the items right away.

Many TF2 players have now started comparing the game to MMO RPG games, where you have to grind for countless hours to get items. People generally play FPS games for immediate action, not hours of gameplay ending in frustration that you still haven’t gotten the item you’re looking for.

Unfortunately, with all the work the TF2 dev team has put into the inventory, drop, trading, and random unique finds like hats, there are not likely to be changes anytime soon.