1 The wind
Wind can affect the temperature, making it hotter or colder; the bridesmaids' dresses; everyone's hair; and whether the ceremony can be heard. Test beforehand; it can be as simple as moving the tables around a corner, or across the park to a more sheltered area.
Plan decorations and tablecloths you can anchor attractively; choose weighted fabrics for the attendants; warn them about the hair thing; and consider the need for a souns system and mike clips. If you're renting a tent, make sure its wind-worthy.
2. The comfort of your guests
Okay, if you're getting married in San Diego you may hit a long spell of "perfect" weather, but failing that, your event may be hotter, wetter, or colder than many guests would prefer. If you're having a lot of out-of-towners, particularly, include information about the weather possibilities on an insert with the invitation and accommodation recommendations. (I went to one in Dallas in July; 111 degrees that day, and boy were the people from Colorado and La Jolla dressed wrong!)
If it's hot, provide iced bottles of water when they arrive, hand fans, a tent with fans. If it's chilly, tent, warmers, a hot beverage when they arrive, lap blankets, portable heaters.
3. Nothing will help if it pours down rain -- you must have a Plan B
4. How tough are you?
Come on now, if you're at all the fussy type, or prone to nerves, this simply isn't for you. You'll have to manage your hair outside, for heaven's sake.
5. The call of the wild
Inclement weather -- or even just ordinary ocean waves and bird calls -- can wreak havoc with still photographs and videos alike. I recall an amateur wedding video where most of the sound was the wind whistling around. There may also be a fountain or waterfall, dogs barking, kids screaming, birds chirping.
I've seen even cloth tablecloths blow in the wind, turning over glasses, candles and centerieces. Wind, bird poop, the shades of night falling... Be extremely realistic when planning your decorations. Actually test them outdoors.
7. Nature red in tooth and claw
It's from a poem, okay, but that's where you're going to be. Don't fantasize about how "romantic" the place is; go take a look. Think realistically about beer bottles and cans on the beach; getting the lawn mowed and weeded; planting seasonal flowers; bugs, flies, snakes and spiders; the sounds of barking dogs and (uninvited) children, or, for that matter these days, teen drugs dealers in the park.
Have a visit with your eyes open. Then do what you need to do, or resolve it's "good enough" and don't worry. The world of nature isn't going to shut down just because it's your wedding.
8. Safety...all ages
You're in love. You're planning your big day. However, you must limit the things you have to worry about if you're going to enjoy yourself. A celebration in a park by a lake would be nice, or by the pool at your aunt's mansion, or even at the beach, but in actuality, if there are going to be children there, and drinking... there's also going to be the chance for a real tragedy.
Which brings up -- all ages will likely be there, and must be thought of. Can you really picture your great-grandmother standing in the surf with her hair blowing, stepping over man-o-wars while picking sand out of her teeth... for long? Can the kiddos go that long without a restroom? Many people can't stand for a long time; others need protection from extreme heat and cold.
9. The food
Do I have to say it again? Mayonnaise (and food items with that as an ingredient) can't sit out in the hot sun for more than an hour or so. Avoid perishable dishes. Wind can make it hard to get the grill lit. Ice cream won't do when it's 90 degrees outside, nor will hot hors d'eouvres last long when it's 50 degrees outside.
10. The sun, the moon and the stars
Yes, he promised you that... and you'll be promising it to your guests, one way or another. Keep in mind the sun moves through the sky, and plan your event (and the photographs) accordingly. One of the most beautiful weddings I went to was planned at a hillside lodge, with we guests arriving just as the sun was setting.