1. Do NOT forget that it will most likely be daytime when your guests arrive! Have cold water available for guests as they wait for the ceremony to start. In the South, at least, it can still be very hot during the day. Make sure you keep your guests hydrated with iced water. If its feasible, provide shaded areas over your ceremony seating and hand fans or parasols if shade is not an option.
2. Do NOT forget to mic your officiant. No matter how loud you think he can project, wind and outdoor noises will drown out his voice...and yours when you're saying your vows!
3. Do NOT forget to have bug spray available for guests. You can tuck cans of it in baskets in the restrooms and in spots that photos won't catch them in the background.
4. Do NOT assume you are immune to wind. Stay away from tall floral centerpieces if its windy outside. They will fall over. So will pipe and drape. Instead, if you want a tall element, use sculptures or tall, heavy cylinder glass with low floating candles and flowers.
5. Do NOT place your cake outdoors. This is the one everyone hates to hear, but just don't do it! If you need bug spray to keep the mosquitoes off of your guests, the cake needs protection too. To prove this point, here is a photo, taken about an hour after it got dark at our last outdoor wedding. Note: this cake has butter cream icing. The Groom's cake was fondant and bugs did not stick to it, but it did sweat. So unless you like your cake with a side of West Nile, keep it indoors.