Not Being Considerate of Your Guests
If your outdoor wedding must be in the heat of the summer, consider setting the time of the wedding in the morning or early evening. If you insist on that late July or early August wedding, in the middle of the afternoon, provide water, lemonade, or other drinks for your guests. You might also add to the invitation instructions for them to dress casually and comfortably. A tent or parasols for the guest will relieve their discomfort as well.
For a drafty, winter wedding, make sure that there are heaters and no drafts on guests. Provide blankets, and hot tea, cider, or cocoa at the wedding to make your guests more comfortable. After all, they made the effort to come and share your day with you. You want them to remember it as a pleasant experience.