for quests, it's hard to say because it kinda depends on how they play into the rest of the game. if they're just kinda to the side then it doesn't make much a difference, but if they're a through-line for the entire game you'd probably want to make sure there's enough content to satisfy people, and at the same time you obviously want to make sure all of that content is compelling. i think runescape does a pretty good job with making neat quests in an online RPG setting. i haven't personally really seen any other game like it that does quests in a more interesting way.
i think there's a place for both with bosses. the occasional powerful field enemies that you bring a party together to fight are cool, but it's a lot easier to make neat levels and storylines if you deliberately place bosses as a kind of climax to a particular section of gameplay. also, i think there's value in players being able to anticipate what's next, esp. in an RPG setting where there's often stuff people might want to do to prepare for a more intense challenge. i'd say the latter would be the primary way to do things since it's much easier to design around.
also end-game players who have already gone through the structured content (or people who are grinding) would probably enjoy taking down more powerful, respawning field bosses. mabinogi is an RPG i'm aware of that's done this
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