My heuristics look roughly like:
~60% of score: does it carry some actual new concept/perspective/piece of understanding of rationality/psychology/the world as pertains to those two?
~30% of score: is it likely to be useful to people in general (not just in a few particular areas of interest/special cases)?
~10%: is it fun/well written/interesting?
[Note: There's already a gather-it-all feed by deluks917, and the lw summary recently had a "most recommended" section, so it covers some of what I'm doing here.]
This is an RSS feed that aggregates the most valuable posts (according to me) from around 40 or 50 rationality blogs. It's relatively uncluttered, averaging 3-5 articles per week.
Feed URL: http://www.inoreader.com/stream/user/1005752783/tag/user-favorites
There's also a Facebook page version, and you can view it online using any of the available free RSS viewers.
Edit: see my comment below for details of the heuristics I use for selecting articles for the feed.