St. Lucia is awesome, as are many islands in the Caribbean. I've been lucky enough to travel to many sites and loved St. Lucia, Dominica, Barbados, St. Vincent... oh, hell, I enjoyed every place I went! Of course, traveling there can be expensive based on flights (everyone has to fly to Barbados and then hop islands on smaller planes from there) and there is a real third world aspect to some of these places as they are no where near as developed as locations in the U.S. (The resorts however, are all fantastic but make sure you get one with air conditioning unless you like to sweat.) If you check into St. Lucia there is a resort there built onto the side of a hill facing the Pitons (the two mountains on St. Lucia) which looked like an idyllic place to honeymoon... it's called the
Anse Chastenet.
On the more budget side of things, the Riveria Maya (on the Cancún/Cozumel side of the Yucatan Peninsula) also has some great spots as John said (of course, you can still find pricey/swanky spots if you want.) Check out Playa del Carmen - it's about 45 minutes south of Cancún and is the ferry point over to Cozumel, so logistically its a good spot. We stayed at the
Iberostar Paraiso Beach (there is a Del Mar side as well, and they were building a third "side" to the resort that should be completed now) and it was all-inclusive and just great.
(Edit: Apparently there are 5 properties that make up the Paraiso complex now... guess it's been a while since I was last there. HA!) This property is located about 15 minutes north of PdC, so you're kind of isolated from both Cancún and PdC - which is nice if all you want to do is chill by the pool and/or on the beach drinking Coronas or margaritas.
Congrats on the pending nuptuals, and good luck with wherever you decide to honeymoon.