This thread is bringing up some very interesting moments.
Esquilax said:
Thief I/II are my all-time favourite games, but can you really call the missions in them, as fantastic as they are, quests?
I think putting that arbitrary line between mission and quest is going to achieve nothing. Calling Thief missions, quests, is perfectly acceptable IMO, even though this thread was specifically stated as being for RPGs. "Mission" and "quest" are conventions, not definitions. The problem is assuming they were designed to be definitions from the beginning, rather than being a part of the genre-based descriptive terminology as they actually are (same way "RPG" originated). I think you will find that if you strip everything else away, what you have left for a mission or a quest is an
objective in a context, the rest is left to designer discretion as to the depth of layering to define that context, including dialogue, characters, worldbuilding, scripted events and other content.
As for memorable quests in RPGs for pure novelty factor, I always liked a lot of the quests in Dark Sun 2. They pulled no stops in including just about anything fantastical element they could think of (with interesting adjustments to fit more into the Dark Sun setting), which while not being a good standard for craftsmanship, do grab the attention of the player really well, especially if you are unfamiliar with many of the tropes of D&D and Dark Sun (I wasn't, at the time).
Investigating the disappearance/murder of workers in the mines was a very atmospheric and sinister mission/quest, even though the mines were pretty horrific to navigate.
Many of the tapestries were interesting as well, including the alchemical processes, the shades banquet thing and the old man who talks in clicks (frustrating, but strangely fascinating).
I should also mention I enjoyed the Bloodmoon werewolf quest(line) in how they all tied together through the dreams and mythology, and really felt a lot better than the standard Morrowind text-database delivery.
As for deep and thought provoking quests, there are not too many of those around. SI did a really good job of structuring its quests with how well the characters are all fleshed out and the inclusion of morals and emotions in so many areas.