4) Dancers like songs of the right length.
You usually don't have to worry about your tunes being too short, but in the rare instance of having a favorite tune that's only one minute long, you can make a double or triple-length version easily, splicing together repeats of that tune, into one longer version, while maintaining the beat. Music editing software is a good way to do this. Audacity is the most popular free cross-platform software for music editing.
The far greater problem is a tune lasting too long, usually meaning more than three minutes. It's a twofold problem. (1) Most social dancers would prefer to have four 3-minute dances with four partners than two 6-minute dances. Variety and contrast are good. (2) Many Leads feel that their repertoire of freestyle dance figures is exhausted after two or three minutes, and they would much rather move on to something else.
Since many recordings are longer than three minutes, you have two choices: fade the song out at about three minutes, or edit out a central part to retain the original ending. I always listen to the original ending to see if it fades out, as much popular music does. If so, then I'll know that an earlier fade-out at 3 minutes is okay.
But if there's a definitive ending (which is especially important in swing and tango) then I'll edit out some central strains to shorten it to three minutes, keeping the original ending. If the song is four minutes long, I'll listen through the piece several times, searching for the minute which I can remove from the center without harm to the song, usually a verse and chorus among many repeats of that verse-chorus. If the lyrics are telling a story, you might worry about removing an important middle part of the story. Don't worry — dancers are too occupied with dancing to be listening to the lyrics. Once again, there are plenty of software options for editing middle sections out of music to make a 3-minute dance version.
Exception 1: Some fast and exhausting dances might want to be shorter than three minutes, like a fast techno polka for instance.
Exception 2: Some dance traditions favor long songs, like salsa. And some groups are so specialized, like West Coast Swing clubs, that the Leads know hundreds of variations for that one dance form, and are happy with longer songs. Good DJs ask what the dancers prefer, including song lengths. Dancers' wishes always come first.
5) Variety and pacing
You want to give your dancers a contrast between high and low energy. Obviously you don't want to play two tunes in a row for the same kind of dance, at the same tempo. Have you ever danced to a big band that played three swing or foxtrot tunes in a row with the same tempo? The bandleaders were clearly not dancers.
Be aware that dancers will want to catch their breath after a fast tune, so maybe follow it with a slower one. And they don't want to be lulled to sleep with too many slow songs in a row. But you don't have to strictly alternate fast and slow dances. Some DJs like to build the energy over several dances, bringing the dancers higher and higher but without exhausting them (yet). Similarly, a string of several quiet dances can effectively set a deeper mood while also building up a desire to be hit with a high-energy set.
The art is to find the perfect pace, without too many fast or slow dances in a row. If you want a safe rule of thumb (which has exceptions), don't play more than one or two really fast tunes in a row, and don't play more than two or three slow tunes in a row.
