What do you think makes a side-quest entertaining?

 Discuss.

It actually being a side-quest for one. I have seen people claim side-quests but it turns out it is within the main story, those are not side-quests IMO. Side quests are things that have a small story, usually about a NPC who lost his daughter and need to find her.

My favorite side-quests would have to be something relaxing, though.

I think if it introduces more lore of the world, or a bit more information about a side character, then that makes it rather enjoyable.

I created one in my game where it is mentioned in passing that one of the characters which you can take as a party member might be getting a bit bored with being stuck in the underground base, and would probably enjoy tagging along with you if you would go and see her bandit companions out in the wilderness.  Doing this provides a bit more of a back story on her character, and shows how much she loves her bandit lifestyle, and cares for her bunch of cutthroats.   :)

I thought it was just a nice way to break up the focus on the main story quest in the middle of the game.

It all comes down to execution. Everyone knows a great RPG needs side quests, but many people just slap gather x item or whatnot quests from random chump enemy, which are not only boring as hell, but they tend to be a drag.

If you're gonna add a side quest, it should be fun, and "rewarding".

If you're gonna have various gather x item quests or whatnot, why not tie it into a bigger sidequest, like a special side quest dungeon, with many objectives, one of which just happens to be the gather x item objective. You see, this way not only are you exploring a new area, but you are potentially completing other objectives, while you're doing find X item/kill X enemy quests.

Most of my side quests are gonna have a different dungeon tied to them, and to make things even more exciting, the dungeons are not even gonna show up on the world map, until you pick up the side quests. There will be tons of exploration, treasure, and rewards for doing them. Furthermore this gives you a reason to have more boss battles/to have a special item drop from said boss, which will make things even more interesting(hm what will I get from this boss now :) ).

Then you have your classics, like a Casino, and Arena, which are always great, and give you yet another reason to give awesome rewards, ect. 

In short they need to be fun, and rewarding.

In my game, I'm also gonna have my take on the greatest side quest that was even made(the Ancient Cave from Lufia 2. However I'm probally gonna add a quick save feature, just for that dungeon), and here's the great thing. My game is gonna be even more equipment centic then Lufia 2 was, plus I'm gonna have many additional types of special items that can be found(For example recipe books, because there's also a crafting system, each Actor has a special type of equipment only they can use, ect)

This dungeon is gonna be my masterpiece, It's a 100 fl dungeon, 99 of them are randomized(in the same way that the Ancient Cave in Lufia 2 is, although I'm not copying Lufia 2s maps, so mine will not only look different, but probally have a even bigger module pool to pull from), and done completely through eventing(lol I'm crazy) :)

To Peridot: Yea that's a good point as well, and yes since the world/universe the game takes place in, is 25 years in the making, I'm definitely gonna have a lot of lore about such in the side quests. Infact one of my side quests involves a secret optional char whom is not only a major part of the lore to the world most of the game takes place in, but her background story plays a huge part in the history of the universe ingeneral.

I played Final Fantasy XI for the good part of... 8 years? Or so... FFXI is mostly sidequests. It has amazing main storylines and plots, also great battles and hunts... but for people like me, who like to explore the world they are playing in, it's all about sidequests.

For this reason I believe a good sidequest MUST have a good story, regardless of how long or short it is, it must feel natural and grounded. They shouldn't feel like excuses for the player to do X or Y... there should be a reason and while the reward must be an interesting one, the fact is that a good sidequest should justify itself. Which is why I think it's important that the world feels alive, each NPC is a "living" being in your world and they have many interests, desires, dreams, goals and hopes, and each sidequest must be as much for them as it is for the player. The player  must feel he is really helping them

With that in mind I say a good sidequest should be also a good reason to explore the world around you, to go into those caves you would have otherwise missed, to thread into dangerous places and feel the adrenaline of having to sneak to retrieve something of great value, at least, to the NPC...

If done well, a good sidequest should give you a sense of wonder about the world the player is in and a feeling of achievement when it is finally complete.  

Not that is any easy to do this, of course.
Best. 

I actually rather enjoy how the Tales of franchise handles sidequests.  Multiple stages with rewards staggered throughout that impact and supplement the main storyline.  IE, Jade Curtiss's artes quest in Tales of the Abyss.  You gather 12 stones and every three nets you a new arte.  The stones are scattered from that point on and believe go until endgame.  It's a preference of course on my end.

To me the most important aspect is execution and purpose. What reason does your character have for doing this? Why are they involved at all? Why did villager number 4 ask the character who he doesn't know to do this thing for them in the first place? In this regard I like having side quests that loosely tie into the main story, maybe for informational purposes or what have you. Better a dozen side quests with decent self contained stories and a purpose than 200 fetch/slay quests while your characters are supposed to be saving the world and are not freelance couriers/mercenaries and have no real reason story wise to be doing any of them.