I did Riviera Maya, just south of Cancun. Easy to get to from both coasts. Everyone said it was the best wedding they ever went to. It was super fun, and minimal planning. It becomes more of a vacation for everyone. It was a small wedding though, about 25 people. Mostly mid 30s in age.
Wedding costs were very reasonable. It was an all inclusive, which everyone loved because they could get twisted and just pass out in their room.