I think it's pretty unfortunate that post of the day is inherently limited to one post per day (and even then, it's often not updated every single day, I'm guessing since just one day on the community beta doesn't give a post as much exposure as Riot thinks is deserved. Not to toot my own horn or anything).
I do see a lot of great content being posted, but there's also more posts that don't seem to be as developed (ie a simple question about the game mechanics, complaints about minor issues in the game, yet another Sion rework idea that's only one unpunctuated run-on sentence long, etc.) that Riot has to sift through. As stated in the introduction to post of the day, the goal is to promote posts that probably won't get the exposure Riot thinks the post deserves. I'm assuming they direct their search more towards posts that don't already have a ton of discussion or upvotes on them, which could be a goldmine of ideas and discussion, or could be one of the posts I mentioned before.
I sort of wish they would also put a date on when the post was promoted to post of the day, that way maybe more than one can be pinned as a post of the day at a time without being confusing (and being more accurate to the name). More content could be recognized, without reducing the exposure time