Introduction

A problem with condensing a complex arena into a few terse paragraphs is that many details must be abandoned favouring a broader scope. You have raised a number of issues, and I would encourage you to think about my response not in terms of absolute answers but in terms of possibilities that, while imperfect, would move us closer to an ideal policy-creation system than what currently exists throughout the world.

Debate Mentality

Where you see Reddit, I see StackOverflow, complete with a system of reputation and moderation. Unlike Reddit where anyone can post a comment, this policy-creation system is akin to StackOverflow where a person must achieve a certain amount of reputation before making a moderated post. With moderators selected at random, for a random interval, there is little chance for hive-mentality to fester.

Scientific Evidence

I understand that you don't see how it is possible to infuse scientific evidence with policy review. Especially given how politicians today seem to shun science. Part of the problem is that there is no centralized location for public discourse on a subject that brings together all vantage points. Fortunately, we can leverage DOI links on a policy while debating it, and include a wiki for each benefit and drawback. Consider:

http://imgur.com/a/PK69j#3

Facts must be backed up by research by linking to a scientific paper using a DOI. Evidence can also be submitted using a wiki-style format, such as:

http://imgur.com/a/PK69j#2

This could include video evidence, calculations, budgetary impacts, and other details necessary to make an informed decision on whether the benefit or drawback is sound. The culmination of benefits and drawbacks are succinctly summarized so that people can still make an informed choice without delving too deeply. (But people can go deeper if they want, assuming the topic interests them.)

Opinion vs. Fact

Part of the issue here is that I think you're looking at the system from "opinions" being presented. The system caters more to facts than opinions, in the same way that StackOverflow emphasizes correct answers over opinions. The example I gave illustrates how this would work: "Some consider marijuana use to be morally wrong." This is an opinion, which is easily countered with: "Enforcing the status quo morality may reflect and reinforce deep-rooted prejudices," with a citation.

http://imgur.com/a/PK69j#3

Reputation

There are a number of ways reputation can be earned in a given subject-matter: For a presenting a fact that gets a large number of up-votes; for graduating an accredited university; for proposing drawbacks and benefits that pass moderation; by attaining up-votes on drawbacks and benefits. Reputation can be lost by casting down-votes or by receiving down-votes.

Institution

I believe you assumed that this system would be initially available to the general public, but that need not be true. It could, to start, be opened to politicians, lawyers, and scientists, who have a vested interest in ensuring the system does not get abused. Effectively as a means to facilitate the communications process around proposing and accepting policies. This would eliminate your concerns regarding a Reddit-style law.

Self-Referential

The system could be made self-referential whereby changes to the system can be proposed using the system.

Conclusion

Pointing out the flaws in a system is a great way to open discussion, and I thank you. Pointing out how the system can be improved to eliminate or mitigate those flaws is even better.