I like quests which actually make you care about the task, and make you want to complete the quest instead of just wanting to get over with it to get the rewards, or the ones which start as relatively simple, but turn out to be something a lot bigger and more significant. Overall quests, that are more like small adventures instead of mindless fetching or killing quests. Baldur's Gate 2 for example has got it right; investigate a mysterious cult holed up in sewers beneath the city - save a town being attacked by wild animals for some reason - your party member becomes aware, that his former wife has given birth to their child after he left, and you got to deal with it - find out why people of a small rural village keep disappearing at night - solve the mystery of people murdered and skinned for no apparent logical reason - a noble hires you to kill a bunch of ogres marauding his lands, but it turns out to be something quite different - find a caretaker for an orphan child, and so on and so on, no "go there, kill chickens and bring me 10 chicken lips-bullshit. And all of these are completely optional, yet full of interesting stories and dialogue, different ways to accomplish them, choices to make and usually proper rewards. I also like (optional) quests considering your party members, which open up their background and in the end make the character stronger.
Overall I think sidequests tend to be a lot better and more numerous in wrpgs than in jrpgs where there are usually very few of them, and they tend to be simple "talk to person A, talk to person B, talk to person C and kill monster D, or just those simple fetching quests. Not that they always suck of course, everything in moderation.