Your elements of a great quest are pretty good, though I would disagree with a few in a sense of quests that wouldn't be considered "Epic."
I think the main things to keep in mind when talking about quests are that it should involve the main characters emotionally. They should have some kind of attachment to whatever the quest is for.
In RPG terms, you have to be able to fail. I don't mean, "Oh, you died fighting these goblins that attacked you randomly on your way to the BBEG's lair." What I mean is when you fight the BBEG, you should have a good chance of failing at stopping his plan or at surviving his assault and after you fail, you should get to see what happens to the world or region or town or whatever.
Foreshadowing is another great thing to add to quests to give them some life. Little things here and there that the players will notice that you've hinted at before and larger things towards the end that they may notice.
Give them reasons to hate and fear the BBEG, but also give them reasons to sympathize with him. A BBEG that is evil for all the right reasons is a BBEG that the players will remember. I always say "There's a thin line between lawful good and chaotic evil." A good example, in my opinion, is Light from Death Note, who kills every bad guy no matter what their crime. His reasons are good and his method is lawful, but his actions are evil and lead to chaos, and the biggest part for me is how likeable a mass murderer can be.
I think my biggest disagreement, Jack, is that the Questors don't necessarily have to be the only ones who can complete the quest, but that is one option. I wouldn't say that it is necessary, but it is a trope to have the Questors be destined to complete the Quest.
I would also say that a Quest doesn't necessarily have to last an extremely long time like years. A good quest could last weeks or months and still feel like a huge accomplishment and still feel pretty epic.