It doesn't have to be half of the posts - if it's approaching 10%, then that's 10% of the posts here being blatantly off-topic. It irks me to no end.
And I'm not sure if you're counting links to stuff about the Singularity, or other such off-topic things.
If a post doesn't at least pretend to be about either epistemic rationality, instrumental rationality, or both, then I don't want to see it on Less Wrong.
LessWrong seems to have two main topics for discussion; rationality and AI. This, of course, is caused by Eliezer Yudkowsky's sequences (and interests), which are mostly about rationality, but also include a lot of writing about AI. LessWrong currently has two sections, Main and Discussion. This is meant to separate the purpose and quality of the post. I think usability of the site has improved greatly since adding the Discussion section. Should split the discussion section, and have one for rationality and one for AI?
Advantages
Disadvantages