More than anything else (it's taken as read that as a commercial photographer you know the technical stuff inside out) you need to be an amiable, sociable and socially aware person to be a great wedding photographer, because there is a balance of discretion and friendly forwardness to be struck throughout the different phases of the day.
At some points stealth and forward thinking will help you get great candid shots without people posing for the camera unnaturally, but at others you will have to cajole and corral a disparate crowd that has other priorities, and with very limited time frames - but you may only get one chance.
It helps to get there early and to identify who the key players are try and strike up conversation with guests as soon in the day as possible - then when critical moments arrive they'll be more cooperative.
Arrive on the scene early and gauge likely settings and shooting positions required (particularly indoors in low light if you need to shoot flashless). You'll need excellent kit, and backups of absolutely everything - there can be no rerun.
Know the locale well and where there are picturesque locations nearby for posed photos - even if you have to briefly travel off site it's worth it, particularly if the main location isn't amazing.