I've noticed that many hosts give a list of rules in their lobby description, and they're all pretty much the same rules. Most players are already aware that they should not do these things, but even so, everyone should be aware of these basic guidelines, so we can all enjoy our custom games more. I've compiled a list of general rules that are applicable to most lobbies. I encourage hosts to link this thread in their future lobby descriptions if they don't want to write out the lists themselves.
Rules
Don't be a douche bag. This is the most commonly stated rule in any lobby. You'll always see it or something like it sitting at the top of the rules list. Be mature. Always consider the integrity of the lobby and the other players while playing. Don't intentionally hinder the progress of your teammates or play the game in a way its not meant to be played in a deliberate attempt to anger your fellow players.
Don't be annoying with your mic. Nobody wants to hear you screaming every time you see somebody or take a bit of damage. Just pipe down a bit, and don't be that annoying guy who takes way more than his fair share of the conversation. Don't blow, sing, or play music into the mic.
Watch your Mouth. A bit of cursing is usually all right, but don't overdo it like a call of duty player. Also, while trash talking is fine, don't continuously insult another player or engage in long pointless arguments.
NEVER be AFC (away from your controller). Occasionally, you'll need to put the controller down for a minute to use the bathroom or something, and that's fine. But players who just walk away from their game for long periods of time ruin it for everyone else in so many ways. It unbalances the teams and changes the player count, and the players often don't realize that you're not playing for a while.
Also, avoid leaving in the middle of a game. Sometimes, a host would be excited to show off a map he made and set the time limit way too long, making it extremely boring to play the whole thing, and it's fine to leave in this case. However, avoid quitting during an eight minute competitive map, as it unbalances the teams. Sometimes, it's necessary to leave immediately, but just be aware that it's frowned upon.
RSVP. It makes life easier for the host if he knows who is showing up, and it also helps make the lobby "look" better, which will likely encourage others to join. If you do RSVP, make sure to show up, unless you've left a comment stating that you're not positive if you can come.
Also make sure to send your hose a message at the start of the game, or whenever you join. It's a million times easier for the host to invite players from their message than it is to type the gamertag of every single player on the RSVP list and send each individual an invitation.
Bring a Mic. Most lobbies won't require this, but it makes it more fun for everyone when you're in the game chat. It also makes it easier for the host to coordinate with their players. Also, make sure you're in a game chat, not a private party. Players in a private party can't communicate with the rest of the lobby, and these parties are frowned upon.
Get Forge Island. It's free. Everyone hates it when the teams are just right, and they load up a forge island map only to find that there's that one dude who doesn't have it. If you don't have this DLC, you may be kicked so the rest of the lobby can proceed.
Rate your Host. While not mandatory, a rating means more than you might think. After the games, hit the check mark unless there is a very specific reason you don't think the host did well, in which case you can rate him down.
Failure to follow the rules will usually result in a warning or two before being kicked. If I missed anything, please let me know so I can put it up.
Rules
Don't be a douche bag. This is the most commonly stated rule in any lobby. You'll always see it or something like it sitting at the top of the rules list. Be mature. Always consider the integrity of the lobby and the other players while playing. Don't intentionally hinder the progress of your teammates or play the game in a way its not meant to be played in a deliberate attempt to anger your fellow players.
Don't be annoying with your mic. Nobody wants to hear you screaming every time you see somebody or take a bit of damage. Just pipe down a bit, and don't be that annoying guy who takes way more than his fair share of the conversation. Don't blow, sing, or play music into the mic.
Watch your Mouth. A bit of cursing is usually all right, but don't overdo it like a call of duty player. Also, while trash talking is fine, don't continuously insult another player or engage in long pointless arguments.
NEVER be AFC (away from your controller). Occasionally, you'll need to put the controller down for a minute to use the bathroom or something, and that's fine. But players who just walk away from their game for long periods of time ruin it for everyone else in so many ways. It unbalances the teams and changes the player count, and the players often don't realize that you're not playing for a while.
Also, avoid leaving in the middle of a game. Sometimes, a host would be excited to show off a map he made and set the time limit way too long, making it extremely boring to play the whole thing, and it's fine to leave in this case. However, avoid quitting during an eight minute competitive map, as it unbalances the teams. Sometimes, it's necessary to leave immediately, but just be aware that it's frowned upon.
RSVP. It makes life easier for the host if he knows who is showing up, and it also helps make the lobby "look" better, which will likely encourage others to join. If you do RSVP, make sure to show up, unless you've left a comment stating that you're not positive if you can come.
Also make sure to send your hose a message at the start of the game, or whenever you join. It's a million times easier for the host to invite players from their message than it is to type the gamertag of every single player on the RSVP list and send each individual an invitation.
Bring a Mic. Most lobbies won't require this, but it makes it more fun for everyone when you're in the game chat. It also makes it easier for the host to coordinate with their players. Also, make sure you're in a game chat, not a private party. Players in a private party can't communicate with the rest of the lobby, and these parties are frowned upon.
Get Forge Island. It's free. Everyone hates it when the teams are just right, and they load up a forge island map only to find that there's that one dude who doesn't have it. If you don't have this DLC, you may be kicked so the rest of the lobby can proceed.
Rate your Host. While not mandatory, a rating means more than you might think. After the games, hit the check mark unless there is a very specific reason you don't think the host did well, in which case you can rate him down.
Failure to follow the rules will usually result in a warning or two before being kicked. If I missed anything, please let me know so I can put it up.