In no particular order:
PPDs
Emotional "scenes"/violence
Drunkenness/use of illegal substances
Not including relationship partners as plus-ones/Inviting them as "and guest"
Insufficient seating
No protection from elements
Tiered hospitality
Couple/hosts ignore guests
Cash bars
Potluck receptions
Cash requests/registries
Registry info in wedding invitations
"Adults only"/Indicating who's not invited on invitations
Gaps between ceremony and reception
Not enough food/drinks
Food/drinks of poor quality
Unsupervised kids
Insufficient/unsanitary bathrooms
Expecting guests to work wedding without compensation
No thank-yous for gifts
Feel free to add.