You don't get to tell your bridesmaids or groomsmen how or whether they will color their hair, do their hair, wear makeup, wear particular underwear, do their nails, go to a tanning bed, diet, exercise, or get any physical flaws fixed. You don't get to have a fit if one of your bridesmaids gets pregnant, she didn't do it just to spite or upstage you.
Don't choose people for your wedding party based simply on attractiveness. Don't disqualify anybody because you don't think they are attractive enough.
If one of your bridesmaids complains about the dress you've chosen, LISTEN. Don't pick something that will be unflattering to some of them, or that they might be uncomfortable in. Some larger-breasted women are not comfortable in strapless dresses, for example. It might also be best to choose something where the style doesn't scream out the decade of the wedding. I looked at the bridesmaids dresses in my MIL's wedding pictures, and they just SCREAM 70's. They're funny-looking to someone looking at them now (or at least they were to me, and I even kind of like some 70's fashions).
If you're having an outdoor wedding in Houston, Texas, at the end of summer, don't make the guys wear tuxes. The average daily high there at that time is around 35 C, and it's humid. My DH was sweating way too much to enjoy his brother's wedding.
If you're having an outdoor wedding anywhere where the climate tends to be extreme in any way, make accommodations. If it's hot, have shade and bottles of water available for guests, that sort of thing. You can still get married indoors, and that might be a good idea in some climates. There was a huge thunderstorm the day of my wedding, but I didn't care, because we were inside.
Try not to have any religious or political stuff in the ceremony that might upset any of your guests. If you've invited non-Christian guests, this isn't the time for a sermon on how all non-Christians are going to hell, for example.