He Refuses to Deviate from a Set Playlist
Your DJ doesn't have to play dirty songs to be a dud. Some DJs refuse to deviate from a set playlist or will charge you more for asking them to do so. It doesn't matter what his reputation is or how good his playlist may be, this is your wedding, and you're entitled to whatever you want.
As we explained earlier, part of the DJ's job is to play a mix that everyone's happy with; anything else is just lazy and unprofessional. This is one of those issues that needs to be brought up before your big day, as most of the strict-playlist DJs don't go around advertising their refusal or inability to mix things up.