Incredibly shaky research will get frontpaged because it resonates with popular political themes, and it receives the full clout of the name of science in the minds of reddit's userbase. For example this thread[0] which was peer reviewed in a journal that is openly written from a "feminist perspective"[1]. It may pass r/science's standards for being scientific but when a journal shares the same biases as the researchers they're reviewing that's a problem.
But if, say, Gerard t'Hooft were to write a great blog post about modern physics it's not welcome in r/science because it's not a peer reviewed paper, even though it probably has far more scientific merit than a lot of stories that r/science pushes to the default frontpage.
[0] - http://www.reddit.com/r/science/comments/22vbpp/barbie_expos... [1] - https://en.wikipedia.org/wiki/Sex_Roles_%28journal%29