My opinion is that "storyline" quests and roleplaying don't mix. Storyline quests usually only make sense if just one person ever does them. But the reality is that thousands of people save the same town over and over again. They get the same speeches, the same experiences, and the same gratitude. Storyline quests are for single-player games, not MMORPGs...at least, not MMORPGs that actually care about roleplay-conducive consistency and logic.
This opinion might not be understandable unless one places it on the foundation of a particular definition of roleplaying: in a massively multiplayer online environment, playing a role is not sufficient - instead, in-character interaction between players, the game world, and each other is the primary factor.
In replacement of "storyline" quests, I suggest live storyline events that have persistent effects on lore that evolves over the course of the game's existence. This isn't a new idea, it's been done in Horizons/Istaria and Ryzom, just to name two (there have been others no doubt, but I'm less familiar with them).
In Istaria, for example, two playable races and a racial capital city were unlocked in events.
The lore on Ryzom's website depicts the playable races' perspectives on what they think they know about their world, not necessarily the truth of the backstory. From there, that lore would've been updated to reflect the state of the races' knowledge as they uncovered fragments of their past, understanding of their present, and inklings of their future. Furthermore, the lore progress would differ between servers.
With live storyline events, your character has unique experiences that they can tell stories about to their metaphorical (or literal) grandchildren. Epicness is justified and genuine, requiring no exorbitant suspension of disbelief. Though not everyone gets to participate in every event, there are always more to look forward to (unless, as in the cases of Ryzom and Istaria, bad companies take over and neglect the game; note however that Istaria is now under the care of a fine company who are already getting their feet wet in running events, releasing more lore, and providing lore-grounded explanations for mechanics changes).
Ultimately, what I want, as a roleplayer and a sandbox enjoyer, is a storyline implementation that fits in the persistent massively-multiplayer environment instead of being an ill-conceived port from the single-player realm.