I'd echo some of the above in more broad form:
1. Motivations/secrets: Compelling reasons beyond the obvious for events.
(e.g. second in command cleric want's the main cleric out of the way and disgraced. he's in league with the goblins to extort money from the main cleric and humiliate him).
2. Other factions: Adding more than "pcs versus single npc group."
(e.g. The cleric has also hired another group of adventurers. They'll be racing the players to rescue the princess...only one group gets the reward. OR A group of gnolls/lycanthropes were aware of the goblin plan. They stole the girl from the goblins and now are working within the extortion plan in one, but don't have the prearranged deal with cleric 2. The goblins are mounting a strike force to get her back...will they ally with the players or compete with them?)
3. Interesting environments: On both the micro and macro level.
(e.g. It's an island...is there a volcano...is it active? Is that a place to avoid or head towards? That's the macro level. For micro level...the goblins live in a mangrove swamp that's difficult for creatures larger than size small to navigate through. They live in a cave that can only be accessed by swimming underwater. The lycanthropes live on a mountain, one that keeps them transformed most of the time because of the cold, but in a cave that they've fashioned to look like a maw, there is geothermal heat from the volcano, and it's roasting, and reeks of sweat.)
4. Interesting combat elements: Monster tactics and environmental advantage.
(e.g. The example earlier of goblins who live in and use baskets. Fights that take place under water. Fights that provide a nice advantage for players...like a player specialized in trip and balance fighting on a long rope bridge, able to trip goblin after goblin, who can only come at him one at a time. For monster tactics, they don't have to be EFFECTIVE tactics, just interesting. For the goblins, maybe they use "shock and awe" where they all run toward one pc, stab him for a round, then disengage and charge a different pc. Lycanthropes might act like their animals...wolves as a pack, tigers as springing from hiding, etc.)
5. Include one bizarre/unattainable/nonstandard reward.
(e.g. PCs find a vein of copper. It's worth a ton, but not easily obtainable. The reward could be intangible like free healing anytime they are able to come to the priest. It could be a treasure map. It could be a goblin servant devoted to them. It could be the cleric's daughter's love (in either big brother form or romantic form depending on her age). It could be knowledge that gives them an advantage later...like a map of the caves below the volcano, or a prophecy that it will erupt on the next full moon.
EDIT...one more.
6. Put some effort into making two NPCs memorable. Don't focus on too many and don't do only one. Players might get overwhelmed by too many, and only one means that the memorable NPC might never be encountered, might be seen as "oh this is the gimmick npc" or might be killed in one round.
(e.g. the goblin leader is very paranoid. he always has a stable of 4 regular goblins around him who are fanatically loyal and always aid another his hits or ac, he speaks in a whimpering manner or shouts commands, he will not go out during daytime, etc... Or the cleric who is a cleric of goodness, but has his faults such as gambling and drinking too much. Make pcs WANT to remember two characters...and remember they don't have to be the main characters in the adventure, they could be very interesting throwaways. To make them interesting, give them motivation, habits, a catch phrase, a prop, a pet, a particular style, a flaw, a behavioral quirk, and give them "screen time" if you can. Darth Maul, while a cool villian, was not very interesting because we didn't really see him do more than fight. Screen time can be accomplished with illusions, letters of communication, the reactions of others to them --goblin minions laugh at their king/are scared to death of him--, personal objects in their rooms, observing battles and speaking --but from a distance -- though that's risky, posession of minions if they have that power, simulacra, etc.)