I am sure Draklar and Waylander will agree that you can not use a quest in open RP where all players finish with the same 'rare' item, or help someone get that one last thing they needed.
I do concede that a lot of quests are single-player oriented and can't exactly be discussed in RPs because each player will manage to "accomplish something big." For us to make it that they can all be discussed would make most quests trivial matters or things like "oh well that's not it, thanks anyway" - not exactly satisfying.
However, I do think quests say a lot about the character and personality of each NPC. Without naming names I can say that sometimes doing a quest can reveal that an NPC has a hidden dark (or light?) side, has a secret, is a little deeper than you think, is in fact quite eccentric, or something else. It is in these respects that they can be used for RP purposes without revealing the exact nature of "what I did for them."
But thanks for getting some clearer explanations out there (karyyu, eldoth, bilbous, and yes you too utm)
Unlikely: rewriting quests to facilitate talking about them in RP situations.
Likelier: making future quests friendlier from an RP perspective.