Mood, atmosphere, ambiance, pacing; fun, excitement and celebration at the end of the evening; this is what DJ services are all about. 

The cocktail hour should be upbeat.  The reception has just started and a long night is ahead.  Family and friends are meeting up, talking and enjoy each others company, maybe having a few drinks. 

At dinner people what to talk and the music should be a little more subdued than the cocktail hour.   Nora Jones, come away with me, some Sinatra, maybe some jazz like Louis Armstrong or Ella Fitzgerald. 

After dinner the mood should transition from a more relaxed dinner atmosphere, to the dance, celebration at the end of the evening.  In between dinner you are going to have toasts, the cake cutting ceremony, the first dance and father / daughter dance, and possibly a mother / son dance.  Right after dinner and before the toasts the pace should quicken in anticipation on the on going celebration to follow.  After the toasts and during the cake cutting, the songs can reflect the cake cutting, like sugar sugar, sugar pie honey, and how sweet it is to be loved by you. 

The dancing / celebration part of the evening should reflect the taste of the bride and groom and their guests.  There are classic songs from every era that every good DJ will know.  The DJ should mix in songs that every age group will enjoy, especially for the first hour.  The DJ should let everyone know that he can take requests, and be aware what the wedding couple do not want to hear (the do not play list).

Maybe there are very groups of people that have traveled long distances, or represent certain ethnicities.  The DJ can play songs that are dedicated to them, showing appreciation for having traveled so far traveled so far to be at your event.