Usually any author/artist wants something along the lines of your thoughts - if anything struck you in particular or moved you, overall. The rule of thumb I was taught long n'long ago, was put yourself in the creator's shoes and think about what you would want someone to notice, then apply what you see. Even if you don't do it regularly, know that most any who get that sort of input, particularly if it's something you're honestly thinking or have observed or felt, will be pretty much thrilled and you will make their day for that sort of thing. (From experience, every time I get a review like that I usually wind up hugging my Eyeore and bouncing up and down and squawking excitedly while yammering at my husband, and then trying to expand upon those things noticed. Even if it's criticism like 'there seems something off in your 'voice' here, I can't quite put my finger on it/or this here seems to not fit in with the rest of the story/style you've got going on and it's jarring'.)