Announcing the last song is key
People want to know that it's their last chance to get down, but more importantly it undercuts “one more song” chants. DJs hate these because the stop time isn't up to them: most venues have time cutoffs so that the staff can pack up and head home. You're likely also upsetting the bride and groom, because most DJs have expensive overtime clauses in their contracts.
Not every crowd likes to dance
It's a dirty secret, but some crowds just aren't crazy about dancing, so contrary to popular belief, not every wedding features a grandmother twirling her bra above her head to the Black Eyed Peas' “I Gotta Feeling.”
You get what you pay for
A good DJ is like a sports car. In a wedding context, you may not be able to use all their horsepower, but what's under the hood is still important. Skills like beat matching and deeper musical knowledge aren't as crucial, but they'll still make a seasoned DJ stand out from an amateur who insists on the "Chicken Dance." The dirty version.