You neglected what's by far the funnest and most important and user-retaining aspect of open world MMOs: player-to-player interaction - be it friendly, neutral, or hostile.
The absolute most enjoyable open world MMOs I've played had essentially no content. Players forged their own content in the form of geopolitics, war, economics, and copious communication (propaganda and shittalking, largely). They felt like a completely unplanned, natural simulation of militaristic human societies from hundreds or thousands of years ago. Probably not ideal in real life, but very fun for a game.
I'm convinced the best MMOs to come out over the next 20 years will have very little content and very sparse grinding mechanics. The magic is the emergent game and meta-game that springs forth from the bonding and strife between gargantuan numbers of human agents.