Pick songs that you're okay hearing and reminding you of your wedding day for a pretty long time to come. We
picked "Just Can't Get Enough" by Depeche Mode for our cake cutting song and I still get giddy when I hear it. I think of cake and cake makes me happy.
Choose songs that are important to you for special dances. So what if you and everyone else on the planet uses "In My Life" by The Beatles for their dance with their father. That song rocks.
Do yourself and your DJ a favor, and only pick one song for the bridal party entrances (one each for parents and family, bridal party and then the couple's entrance is fine). This really doesn't take as long as you think and it is weird to hear a brief snippet of music after another. It helps the flow of things.
If you're going with a more modern cocktail hour playlist, please pick songs that don't have a ton of instrumentation and that guests can talk over. You don't want it to be a competition.
Pick a short song, around 2 minutes and 30 seconds for your first dance song. Unless you work with a choreographer and have something set, this is a beautiful but pretty awkward moment and I highly suggest you be in charge of its length.
Pick fun songs to compliment other things happening during the reception! We had a late night hot dog bar, so we played "The Hot Dog Shuffle" while they were served. We had Eric Clapton's "After Midnight" play at 12:01am and we had a ton of theater people attending our wedding, so we threw in some good musical songs that got everyone on the dance floor.