Here are the options as I see them. This list should not be seen to be exhaustive.

1. We rely solely on the karma/voting system. The posts which are on-topic are those which are upvoted - on-topicness is thus a quality determined by the number of readers who believe a post is on-topic and of sufficient quality (either in and of itself or by virtue of the attached comment thread).

2. We rely on the Editor's discretion. Those which are on-topic are the posts which he decides not to hide from the feed.

3. We make the feed more dynamic - something akin to reddit's front page schema, where you can filter your front page by subreddit, hide posts from a certain user, etc.

Personally, I would love for #3 to be implemented - but LW may not be big enough for it to be effective (e.g. we have no "subreddits", only tags)

In any case, I do think we need to state our policy. As it is, I Eliezer's plan seems to be to make up policy on the fly and introduce what I think are rather ad hoc (not to mention ineffective) "vote up if you would approve of this" comments.