The 217 people who gave it five stars disagree.Happy people generally don't give feedback.
I think the extreme opinions come either from fanboys ("ermagerd El Capitan is so purdy FIVE STARS ! ! !") or from people who are pissed off about one particular aspect ("my folders won't stay on top"/"how the hell do I work with a RAID now"). I'd rate Yosemite 3.8 and El Capitan 3.6 (not really, but giving an example) – as far as App Store is concerned that means four stars. In other words, the ratings are 100% useless. Perhaps some of the comments would be useful.