One demonstration of the characteristics of useless djs is them not caring about the party guest response to the audio being played.
Another thing for the entertainer to avoid is too regular playing of audio they either like or a familiar set over the clubbers' vinyl taste which is often accompanied with failing play of requested tracks. In addition though an entertainer can also be a bad one simply due to the collection of tracks they have available. This can contribute to the other characteristics of bad disc jockey behaviour. An awful disc jockey usually causes a party to be ruined. This will cause the guests to party less. If the disco is a private one then the revellers will stay a bit to be polite to the host but will always leave by up to 2 hours sooner than they would have done with a better disc jockey. This will cause to the party being remembered as a washout and the disc jockey too. The disc jockey will never get a good reference or any spin off parties as a result. If the party within a night club context the venue would also have detrimental impact on its reputation. In addition the awful entertainer means that within the pub context less amounts of alcohol will have been bought. A happy client will purchase more from the bar whilst at the venue and remain there longer to obtain further alcohol. Only about a week ago I observed a situation where in a wine bar a couple were purchasing alcohol and requested a number of tracks which the disc jockey made excuses for not playing despite the fact the small crowd there were not paying much interest in the music anyway, which resulted in the couple leaving dissatisfied, whom had they remained there would probably have bought further drinks.
 On the night of the gig you should look out for the "trend setters" that the crowd follow. It is recommended that the music played is that they prefer, although of course requests are essential too, and if it is a private party the host's favourites must come first. The timeline of the gig is important too. In general the loudness and style of music should vary for different stages of the party. The early part of the evening will usually be the reveller's conversation time so it is important that the tracks are not too loud and are the gentler ones within the styles enjoyed by the revellers. I have observed a night club clear because loud, heavy fast tracks were performed too early in the evening. An hour and a half later the environment will change as more people have had a few drinks and plenty of chat so will begin to want to get on the floor, from then onwards the records can get a harder and higher in bpm. Generally it can be said that the party vibe needs to be progressively wound up. This recognised flow of events can be different though if the party has a meal involved. In that situation the party may need to be wound down with quieter and gentler vibes again as people usually chat again whilst eating. For birthday and wedding parties the winding down and up of the party may also be essential to fit round speeches. One very essential piece of advice though is if the dance floor has been made empty for any reason which could be due to a speech, meal, unexpected situation such as a fight or even a poor song being played, the party can be rescued with the use of a familiar party song such as "Cha Cha Slide" or a cheesy well known Bee Gees disco number.