NOT HAVING A RAIN PLAN. Yes, ideally the weather will be perfect and you will be able to have the dream wedding you have spent months planning. However, not having a rain plan will cause you more stress, worry and problems on your wedding day. Also, if you are able to come up with a plan beforehand, you will be able to mentally process what will happen if there is bad weather.
TIMING. It's best to not have anything scheduled during noon during the summer, especially pictures. Do yourself a favor and find out the best times to be outside during the hot summer months.
ATTIRE. Planning an outdoor black tie wedding is unfair to your guest. Yes, you can have an outdoor option for your black tie dressed guests, but make sure you are not setting your wedding up for failure. If all of your wedding activities are outside, be prepared for your guest to have more of a laid back attitude when it comes to their attire.
NOT HAVING ENOUGH HYDRATION STATIONS. If your ceremony is outside, maybe have a little lemonade and water station for your guest. Keep in mind many guests get to the wedding ceremony at least 15 minutes before it even begins. So even if you have a quick ceremony, you are still asking your guest to be outside for at least 30 to 45 minutes.
BUG SPRAY. Sounds silly, but trust me it will make a world of difference. Guest will appreciate that you have thought about them and your guests will have a much better time if bugs aren't flying up their noses. Nothing can kill wedding fun faster than uninvited critters.