I cannot stress enough how important quests are in making an average game extraordinary. To do that games need to follow four simple pieces of criteria I always find myself using.
1. The side quests have to play a relevant role in supporting the main quest, whether that is subjugating rebel tribes in order for you to ascend to the throne, or simply helping you to build your reputation in the provinces. In either case those side quests have to impact the main quest in someway to give it meaning. Otherwise completing those side quests can become a tedious chore, which most people will only do because their either completionists like me or want to gain some extra experience.
2. All quests have to fit in line with what the character's role is in the game. A bad example of this is Fallout 4's Minutemen quest line, where you become a General, a rank where you would assume all you would have to do is dictate what needs to be done and assign duties. But instead we have repeat quests of you rushing to defend a settlement, rescue hostages, and clear settlements. Which would make sense for the first few settlements, because you're rebuilding, but once you have three fully functioning settlements, you should be able to use to some type of map to have your followers do some work for you. I mean what's the point of being a leader, if a lot of your time is spent doing the same stuff you already do everywhere else in the game?
You see the same problem appear in Dragon Age Inquisition, you're the inquisitor, a person with great power, and yet you are constantly reduced to a mere servant. Collecting and killing a certain amount in nearly every single side quest, except for friend quests. While the map is an excellent addition to the game, which I hope future BioWare games use, why are there only three people to assign tasks to?Surely the Spy, Commander, and Adviser all have hundreds of people under their command, yet we never get to use more than three. Why?
3. Make every side quest unique and interesting. In all honesty I would prefer to have fifteen good long side quests, rather than having hundreds of filler quests, where the only purpose is to waste your time and keep you occupied. Witcher 3 again is an excellent example of side quests that are not only useful for experience but excellent to play. It's the only game where I actively go around searching for side quests, because I don't want to miss out on anything they have in store for me.
4. The last, and most important, rule for all quests is the pacing. I hate it when a ton of quests are dropped on top of me in the first few minutes of the game. Way to make me feel the weight of the entire world resting on my shoulders! It would make so much more sense for those games with only a few good side quests to spread out the quests to places the main questline will eventually take us too. There's no need to be immediately inundated with quests that make us feel like we are being steam rolled.