Here you will find a list of social settings that are practiced and encouraged at milongas around the US, as I have not experienced it outside the US. I'll update this information as I ask around from people who have danced all over the world. These guidelines are distinct from ones in Buenos Aires, primarily from a cultural difference position.
This list is not in any sort of priority, they are all important, especially when it comes to Tango Festivals. Exceptions to the etiquette usually involve a sense of danger or major discomfort. Without either of these, suck it up and smile.
~ Always introduce yourself, and reciprocate introduction, unless for certain you know the other person and/or they know you, when in doubt, introduce. I am notorious for forgetting names of people I meet in a very short time, and it's embarassing, but it's more embarassing if you don't deal with it in the moment and they catch you.
~ Avoid gossip(unless it's with good intention). This is meant to at minimum, be with your close friends. Reasons for gossip are probably listed under my "Tango Pet Peeves" Section.
~ If you are intending on asking someone to dance, do it within a minute, unless you have just danced with them. If you converse too long, they may get asked by someone else, or may lose interest. If you are committed to conversation, make that expectation known so it can go uninterrupted.
~ Dance with other people. If you came with a partner, you are certainly obligated to your partner, but remember this is a social dance. On and off the floor. If you just came to dance with your partner, you can do that in the privacy of your own home and music. Or set up your own performance gig.
Basics of Tango Etiquette
~ The most common way of asking people to dance is by verbally asking. Some people are dedicated to using "cabeceo" or nodding (non-verbal) to ask people to dance, but this is just something to be aware of.
~ Unlike many other social dances, you dance tango for several songs at a time. Dancing Tango are done in tandas. A tanda means "series" or in tango dancing a "set of songs." This is usually defined by the number of music played in between cortinas. Cortina means "curtain" or a "break" in between sets. Tango tandas are anywhere between 3-5 songs generally and Vals/Milonga tandas are most often 3 songs, occasionally 4. Tandas are determined by DJs. If there are no cortinas, which occasionally happens, then try to stay within 3-4 songs.
~ Never say "thank you" after each dance. You only say it at the end of the tanda, and more specifically, when you are done dancing with your partner. Saying thank you is a message that states you are completely done dancing with your partner and may actually be considered an insult. Under uncomfortable circumstances can this be used to end dancing in the most polite fashion.
~ Hygeine is extremely important. Shower, use deodorant, brush your teeth, eat mints, mouthwash, etc. Some people can suffer from a bad smell. Using cologne or perfume is just a mask that doesn't get rid of anything. Too much cologne and a bad smell combined makes for an even worse smell.
~ Dress appropriately. Tango is classy. If you got the body, contain it. If you don't got the body, contain it. On the other hand, if you got the moves, flaunt it, without performing or showing off. No one needs to see bare skin in sections uncalled for. Similarly, no one needs to suddenly notice "see-through" outfits when there's an exposed lack of clothing underneath. It's just distracting and can bring you the wrong kind of attention.
~ When guests are around, introduce them to people they can dance with. This is highly appreciated. Don't just point people out, it is very difficult sometimes as a guest to be able to even start a conversation with people to try to get a dance in. Certain places have cliques that do not stray from their group, this is clearly noticeable when the guest suddenly feels like no one acknowledges that he's new to the space or is a rare occasion to the space.
~ Line of dance is usually counter clockwise in an even circle/rectangular dance floor. Even if the floor is oddly shaped, maintain line of dance. Do not dance against the line. Allow yourself 1 step against the line of maximum space. Otherwise move forwad with the line.
~ When entering an already active floor even if it's the beginning of the song in the middle of a tanda, it is effective to visually catch the attention of the leader dancing in the outside lane to let them know you are entering. It's like asking permission. Being cut off in line or in traffic is very annoying. This respects the flow of the dance floor. Sometimes you have to wait for the next one if the leader is not paying attention. Do not just jump in, that's for the beginning of the tanda.
~ Pick a lane and stay on it. The most dynamic lane should be the outside lane. No zigzaging or switching lanes. Flow with traffic, it is not a race.
~ No passing or overtaking. This is absolutely important, it may challenge you to figure out how to dance in a tight space that is barely moving. You might find yourself tailgating the couple in front of you. There may be an obvious exception, like if someone is on your lane who are complete beginners and don't know anything about dancing tango socially.
~ No parking. Especially in the middle of a dance. You will be disrupting the flow of people behind you who will start dancing. Keep chatting to a minimum in between songs, I'd say 30 seconds or less. Doing this may cause people to pass or overtake.
~ Protect your dance space and partner. Keep a reasonable distance to the couple in front of you and if you are being tailgated, allow them to pass you, most conveniently when you are by a corner.
~ Forget showing off. Much of this usually means you have an ego that needs to be recognized, when it's not welcome. This is distinct from having fun, quirky dances. Show-offs monopolize the space with no respect for the other dancers thinking they are some hot-shot dancer. Often times never with the music, and all about athletics and performing bad habits really well. It may be showy to the inexperienced dancer, but it's all sorts of wrong to those who have been dancing a while. Consider some dancers you are showing off for are not impressed because they can do it themselves and are there to social dance, not perform.
~ For whatever reason you or your partner gets significantly injured. Slips, falls, stabs with tango heels, bumping into someone. Escort yourself and your partner quietly off the floor. When something bad happens, it sours the rest of the dance already, and you can try to redeem yourself, but it may come with more consequences.
There is definitely more to add to this, and feel free to let me know if I am missing anything. I want to make this list as complete and informative as possible.