Unrealistic goals need to be reconsidered. And to avoid blindness, every goal must be considered allowed to fail.

The creation of good leaders is every bit as part of human nature as the corruption of bad ones. Trying to wish away that fact is futile - and wishing away is all that's happening here.

For meaningful discussion, some form of top-down moderation absolutely is required. That does not mandate imitating any particular other site's manner of top-down moderation, however. So ask yourself, what about top-down moderation is harmful?

If you want to ensure that no central party can cause harm, the only possible answer is for each cluster of users to host it themselves, e.g. like Diaspora - and likely also exchange the data over something like Tor to prevent ISP-level meddling. You can't just pretend that any single host or network is infallible. (And related - currently, on Hubski, whoever is the first to post a certain link gets unquestioned moderatorship over that thread, which is clearly worse than subreddit moderators who have to post at least some sort of rules).

If you want to create communities under the fundamental requirement, there has to be something better than following users (who won't share everything I want to read) or following tags (which offer absolutely no moderation). Think about how small communities work on Reddit - someone has an idea for a subject, they create a new subreddit, act as its (usually sole) moderator, people post/view content there. Since the moderator only has localized power, corruption is minimal, and since they do have localized power, abuse is also minimal. To expand from that smallness, remember that there are often multiple small communities with a similar focus, with some number of overlapping subscribers, but completely unrelated moderators.

To actually produce a workable system, we need something like: every user is the moderator of their own personal subreddit on every single subject, but anyone can post in that provided they have appropriate ranking (settable per user+tag to either "whitelisted" or "not blacklisted"). Allow anyone to add tags to any posts, but the tags will only be relevant to people who (directly or indirectly) trust them for that tag - have something similar to "upvoting the fact that a tag applies". Do not make it possible to follow users, except in a specific tag (but do provide a standard tag for "follow me in this for when I start a new tag"). But do have the sense of tag relationships - perhaps a "suggestions" stream from any particular stream you're viewing.