Steam

If people could post neutral reviews, Steam would still display "X% positive". Nothing would change. Hell, you could let people post neutral, negative, or double plus negative reviews, and at the end of the day, the result is still "X% positive".

If you want any value from the negative reviews, you have to read them. Adding other options doesn't really help. I'm glad people post negative reviews to mention how "blah" a game is. It highlights the review so I can read to see if their problems with the game resonate with me.

In most review sites that have stars, everyone just posts a 1 or a 5, anyway. Or you have those idiots that say "it's perfect, I love it! 4 stars". Then you have those unicorns who post a 3 and really defend it, but it's so rare.

Board Game Geek has an interesting review system where it's 1 to 10, but the numbers are explained with words. This results in a 7 being a really good board game. All review systems need something like that. Words to articulate what the numbers really mean to calibrate people's use of them. Either that or keep it simple like Steam does.