It's EXTREMELY simple:
A good anime review can separate themselves from their personal taste. Even if they personally found it extremely boring, they still have enough understanding of the people who like those sort of shows to identify what the strong points of the show would be to someone who does.

For example, I can't stand CGDCT anime, so I honestly wouldn't be able to tell you what makes Gochiusa such a cult classic compared to similar shows (even though I've seen it). A review that just says "this show is boring, I slept through it" wouldn't tell anyone anything besides I don't like the genre as a whole.