Just some food for thought: I was recently brainstorming ideas for building a more decentralized moderation system, and one of the ideas I arrived at was using the rules themselves as part of the flagging system.
It would work like this: When you flag a post for breaking the rules, the community's guidelines will pop up. You are then asked in this window to highlight the relevant section or sections of those rules that this post has violated. And I don't mean just "select which rule was violated", I mean "use your cursor and highlight the text of the rules that were violated." (with support for highlighting multiple sections if so desired).
This serves the following functions:
1. Communicates why something was flagged (obviously).
2. Forces the person who's flagging the submission to actually read the rules.
3. The subjectivity of the highlighting system is used to make Sybil attacks more obvious. I'll explain why after this list.
4. It differentiates flagging from downvoting. Downvoting is for saying "I don't like this". Flagging is for saying "This violates our community's rules".
As to why this helps reveal Sybil attacks: There are several subjective points on what, where, and how people will highlight rules. Should punctuation be included or not? Should the key word in the rule be highlighted? The key sentence? The whole section? What about examples? Should we include them? Or only highlight them? Users operating in good faith will cluster around common points in common areas, but will have different ways of doing so. So, if a block of users all have: the same input, in the same way, clustered around the same time, then it was likely a Sybil attack.
This system doesn't require that it de-anonymize the people who submit flags, but it does provide a form of publicly visible transparency as to why something was flagged.
Edit: I forgot to make clear, you would be able to see a heat map of the rules that were highlighted for a flagged post.
I'd be interested to hear any thoughts on this idea.