Rain is not a big deal, but freezing rain sucks - when ice forms on all exposed surfaces gears/brakes/and pedals get really slick.

You basically have to keep braking constantly and changing gears constantly to keep moving parts free of ice, not to mention the bike abuse from salt.

This being said - I have ridden several times in freezing rain - I try and avoid it, but sometimes there is no choice but to ride in these sub-optimal conditions. I just go really slow.