What makes for a "strong" LoR for graduate application? I searched this site and it seems that some individual cases were addressed, but is there any general guideline? For example, between a renowned professor who may barely fill up one page, and another professor freshly hired from a postgrad who liked my course project and has more good things to say about me, whose LoR will be evaluated more highly by the committee? What (reputation, length, relevance of area, writing skills, etc) matters more for an LoR?

Thanks!
That is highly dependent on the thoroughness of the committee doing the ranking, and other random facts (i.e., one might hate the guts of the renowned colleague, or have met and been well impressed by the fresh professor). In case of doubt, I'd go for someone who can really talk about you (worked together, took classes with them, ...) than some star who at most will say they've met you and you seem OK.