Not surprisingly the idea of continuity in music was around a long time before the DJ came to use it. Quite simply put music continuity is a harmony created between different scores of music or more recently tracks. By keeping some aspects similar it makes the transition between the songs both easier and reduces the impact on the people listening/dancing to it.
Now imagine your up dancing to one of your favourite Motown classics because you like this type of music and your expecting another to follow. But someone has been and badgered the DJ and he decided to play a high tempo clubland song. Firstly your not going to get a seamless mix from under 100 bpm to over 140 bpm. The key will probably be different but worst of all, your probably going to sit down now as this is not the music you got up to dance to.
A good DJ will try to take you on a journey easing you from one style of music to the next with linking tracks and songs that they know how to mix well. This is why most DJs will both be watching what is going on with the dance floor and advise you when your likely to hear your request. Just saying play my song next is a liberty that only the client should have as its their party and they are paying.
Its true to say a good DJ set is a journey and when talking about a club DJ who will be working their own set they will choose the songs and transitions and practice just this set. This is a liberty the mobile DJ does not have for two reasons, firstly because of requests and secondly because of the direction the client will often provide. In DJ circles the club DJ will often scoff at the mixing skills of their mobile brethren, however they have no real idea of the complexities that are presented with a fluid music selection and having to decide transitions on the fly.
Performing as many events each year as I do each year I get to see both sides, I can honestly say the only events I feel work well with a high volume of requests are theme nights. Once you take away the DJs leeway to transition well, you get more a jukebox/ipod playlist feel to the music, the dance floor reacts to this in waves and you never seem to get a flow and consistency to the dancing.
Andy P