expression
When many moons ago I was learning photography I was taught Pose, light, expression!!!
Pose: you cant always control this element, but when you take control make sure to take the time to pose the Bride and Groom to the best you possibly can.
Light: you can control this all of the time, no excuses please.
Expression: sometimes you can, sometimes you cant when you can, make them smile. Here endeth the lesson!
ENJOY!