Personally, the only thing that makes side quests interesting are good stories. I don't care what they make me do, or if they make me do it a 100 times, if there's a good (story wise) reward at the end, I'm fine. Obviously, it's impossible for a game to have good stories for all side-quest (especially on big open worlds), but the more interesting stories you can have, the better.
This is probably why I don't like MMORPGs (the ones I have tried at least) but I love Fallout series. And I'm currently playing The Witcher 3, which so far has the best side-quests I have seen in a video game.
In general though, I prefer shorter games, since I think my "philosophy" of side-quest stories are more of an exception rather than the rule.
Nintendo and PC gamer
