I'm not asking for some strict formula, more some ability for people to describe what kinds of blogs they're after in more detail in a way that's flexible, but not overly fiddly to use.

I'd possibly put a shared blog under a similar category as the ones with weekly roundups. Which is exactly why I threw out that example - I don't want hard-and-fast Google-style rules, because I would expect that to work out about as well as hard-and-fast Google-style rules ever does.