Having played for many weddings, here are some points to ponder:

Please be considerate of elderly guests: Are you having a lot of walking and are there a lot of stairs?

Pay vendors before hand or have checks ready the day of the wedding. If the later, give the responsibility of delivering payments to someone else in the bridal party. You don’t need to be searching for your checkbook and pen during your wedding.

Do you have a lot of children in your bridal party? Use a decorative garland for all of them to hold onto. Place the oldest children at the front and the back.

Outside
Please inform your guests that you will be outside so they may dress appropriately
Weather: Do you have a back up plan? (If it looks like rain, please move inside)
If it’s rained or been foggy, please wipe off the chairs for your guests
Wind: How long is your veil and is it attached well?
Are you having a unity candle? If so, use hurricane glasses to protect them. (You may also opt for a sand or rose ceremony) 
Flowers: Weight the base and try not to put them on a light weight pedestal
Runners: wrap bricks in wrapping paper or decorative foil and place them down the sides of the runner
Bring sun block and bug spray
Electricity: Does any one need a light or a microphone?
Be aware of the ground: Is it solid and flat enough for tables and chairs? Can guests walk comfortably (in heels)?
Bathrooms: Are there any near by?
Sun: Where is the sun going to be at the time of your ceremony? Will the guests and photographer be blinded? Is there any shade for musicians and elderly guests?  Will daylight savings time effect this?