Sometimes it isn't possible to point to something and say "this is good and here is why I liked it". I've done a lot of reviews on another site, and in some cases I got so bogged down in finding errors that I completely missed the point of the story. On this site, I read the whole post through without trying to find anything. Then I open a text file on my desktop and make notes of my impression.
Then I go back and look at structure, syntax, punctuation, and grammar, making notes in the text file on this also. The primary reason for the text file is to remove the "must get to 50 words or I won't get points" temptation. Another reason for the text file is that I sometimes take quite a while to review. Before, when I was making notes in the review box, I'd find myself getting logged out and have to start again. The text file lets me work at my speed and paste it into the box later.
If there are any 'rules' about doing reviews, I would also like to see them as I am looking to give better reviews also.
There are times when a simple "I loved this!" works very well indeed.
~Tom