It seems that there are still some DJs out there that don't know these simple rules when playing music for dancers. (Disclaimer: I'm talking strictly about playing music at events where mostly dancers attend and solely for dancing, like Salsa/Swing/Tango/[insert your dance here] parties.)
1. Do not play music too loud
This is not a rave or death metal concert, dancers value their hearing and are hoping to use it for a long time to come. More experienced dancers like to chat while they're dancing, and it's very annoying having to shout from the top of your lungs over a blaring song. It's a social gathering after all, allow people to socialize.
2. Do not cross fade dance music
This is a huge pet-peeve for more experienced dancers, they know the song and hoping to execute a finishing move (not the Mortal Combat type) on the ending accent. In fact, leave a 1-2 second space between songs. This will allow dancers to get off the floor, find a new partner, wipe their brow or get a drink of water without missing out. I can accept when real DJs seamlessly merge multiple tunes together or enhance them by adding percussion, as long as the song has a finale. So for the love of all holy, please uncheck the cross fade option in iTunes!
3. Do not interrupt a song in progress
This is a really bad one, a rookie behaviour. You better have a VERY good reason to just stop a song, like an emergency or malfunctioning equipment. Even if the floor is empty and people are hating the song (see #6), at the very least fade it out. If someone is dancing, keep playing.
4. Do not talk excessively over microphone while music is playing
This is a dance party, not "come-and-listen-to-me-talk" party. Not only you're spoiling the dance experience, no one can make out what you're saying anyways! Run your announcements during the breaks between songs (see #2), it will be a used as a rest break and people will actually pay attention. This one is no-brainer, really, yet people are still doing it.
5. Avoid "endless" songs
Unless dancers are training for marathon, they will not enjoy a song that is more than 5 minutes long. Especially a fast one. It's tiring, it's boring and it makes it difficult to leave/change one's partner without being rude. Medleys are good for listening, not so good for dancing.
6. Watch your audience
So you've heard this song at NY Salsa congress five years ago and think it's awesome. Now if you were paying attention you'd notice a blank/confused/unimpressed expression on our faces. Make a note of songs that fill the floor, make an extra special note of songs that clear it. Adjust your play list accordingly.
7. Target your demographics
A good DJ knows his music and what music various groups enjoy. Don't overuse tunes with neck breaking speed at dominantly beginner events. Don't play a lot of Cumbias and Merengues at events where advanced Salsa dancers are in majority. Keep your play list flexible and tweak it on the go as needed.
The first five are the equivalent of "deadly sins" for a dance DJ, the last two are much more common and hint on inexperienced, oblivious or conceited DJ.