1. Weather: If it's too hot or too cold on your wedding day you risk the chance of your guests being miserable. Umbrellas and heaters are a great way to help avoid this issue. Some other simple, but fun, ways to help steer clear of this problem would be to offer scarves and/or water bottles for the guests. They will appreciate that you thought of their comfort on top of everything else!
2. To Much Time Between Events: The last thing you want is for your guests to be bored at your wedding. Pictures are a very important part of being able to remember your day, but it's called cocktail HOUR for a reason. Your guests will get antsy if their forced to wait to long. A great way to avoid this is to get as many photos done prior to the ceremony as possible. First looks between couples is becoming increasingly popular and adding some fun photo opportunities to the day!
3. Bad (or not enough) Food: You do not want the one thing your guests remember about your wedding being how horrible their meal was, and people will remember that. Ask your wedding planner or your venue if they have a preferred list of caterers they've worked with that they know are excellent and always taste a caterers food before you hire them. They are happy to have you come try their food and give them feedback. Caterers are great about working with their clients to create custom menus as well. Also make sure you and your caterers have accurate head counts for the day of the wedding. This will insure they have to proper amount of food and everyone gets served equally!
4. Kids Taking Over the Day: We all love kids (well most of us) and many people these days enjoy having them come and help celebrate their big day. What you do not want is the kids running the show. Giving them their own special table with things to entertain them (coloring books, bubbles etc)and hiring a babysitter to watch them will keep them from running wild at the reception and give their parents a chance to enjoy the event as well. It's also ok to not invite the kids. Most parents love the excuse for a date night!