I have always said that my main job during a wedding is to help the service go smoothly - to calm the frazzled nerves of people who don't remember when they're supposed to do what.
Part of that is showing up on time (my first fear of weddings is that I'll show up late - the other fear is that I'll call the couple by the wrong names - that's why I have the service copied with the names filled in every time I'm supposed to call them by name).
Another main job is to keep Christ central - the wedding is a commitment to one another, but it's also a commitment to Jesus Christ.
I have to add to my list, however. Reminding the wedding party to "flex their knees." I had a groomsman keel over yesterday, right after the couple said their vows, all of a sudden, everyone hears a huge THUMP as he hits the ground. He later told us that the last thing he heard was me starting the vows with the husband... we went through both their vows before he hit the ground.
There was an EMT, a doctor, and a hospice nurse who all helped him out, and he ended up OK. But we'll all remember the excitement.