Wedding planners get an incredible amount of insight into what wedding guests don't like simply by listening to what goes on around us during the wedding weekend. Even a well-planned wedding can be unpleasant if the couple doesn't plan ahead for some necessary contingencies.
Certainly, every guest has a different opinion, but, in my professional experience, there seem to be four complaints that are the most common.
1. Long Wedding Ceremonies
Unless you're abiding by a particular religious tradition that includes unavoidably long ceremonies, aim to keep the length of the actual wedding to no more than 20 minutes. That's how long your guests will stay engaged and interested. And if it's hot out, that's how long they'll sit sweating in the sunshine before they become slightly grumpy and a tad irritated. Longer wedding ceremonies are usually, well, longer, because of long-winded officiants. With the popularity of using a friend to marry you on the rise, brides and grooms risk subjecting their guests to lengthy anecdotes or readings from the officiant, rather than a concise blessing on their future marriage.
2. Problems at the Bar
In my professional experience, I don't recommend cash bars at weddings, if for no other reason than many guests won't expect that, or come prepared with money to buy their drinks. If you've invited them to a wedding, the hosts are expected to provide food and beverages for the duration of the event. When you can't afford to host a full, open bar, I recommend offering a limited bar with only beer and wine. Also keep in mind that an understaffed bar can create long lines that distract guests' attention away from the wedding festivities.
3. Dead Time Between Events
In my opinion, guests don't like to attend an early wedding with a late reception, especially if they're from out of town. This usually happens when the couple has to fit a certain time slot for the ceremony at their preferred venue, but they want an evening reception. Unfortunately, the downtime rarely gives guests enough time to go do something interesting, but it's long enough that most people won't want to sit around in their fancy clothes. That means guests go home and take off their wedding clothes, only to do it all again an hour later. It kind of wastes an entire day for your guests.
4. Bad Performance Art
Nobody likes to watch an intoxicated, off-key wedding guest belting out a song on the microphone at the reception. It's not karaoke, it's a wedding — and the audience is held captive no matter how awful it may be. Fight the urge to let a helpful guest sing you down the aisle, no matter how much they want to, if he or she isn't a good singer. Warn the DJ — in advance — not to let anybody hijack the microphone for a "special" tribute. Save that stuff for the after-party.