I did sound for a wedding a few years ago, and it was pretty bad. The dj wanted to be cranked up (this before the "dancing" part), and didn't get that he was just meant to be audio wallpaper initially, background noise that people could talk over. He accepted my lower level for him, though. Then the band (a sloppy rock-n-roll outfit, friends of the groom, really good band to see in a club, actually) were too loud. The room was huge & echo-y, with tile floors and lots & lots of glass windows, including a huge window directly behind the stage. The band just couldn't play quietly, and I ended up pulling everything out of the p.a. but the vocals. The wedding organizer still came up to me & asked me to "turn down the drums." If the drummer could have played quieter than everyone else could have turned down, but he was a basher & could play no other way.
Anyhow, I felt bad for everyone involved. The bride & groom didn't mind too much (he's a great guitarist & songwriter who's been in a couple pretty big bands, and he understood the situation), but I still wish a bit more planning had been put in place - just thick curtains behind the band would have helped a lot. "Wiffle sticks" (plastic brush drumsticks) have saved me at a couple quiet gigs since then - give 'em to the drummer & have him play normally, and it just ends up quieter. Tone is compromised, of course, but it's worked for punk bands playing in quiet coffeeshops. Unfortunately, I didn't know about those sticks at the time of the wedding.