Reviewing is really nothing more than having a polite, upbeat conversation with a friend. As you would in that case, keep your criticism constructive. Also, keep in mind that creators will be responding to your comments, and their input will determine your reviewer ranking.
Put yourself in their shoes: it takes guts as an artist to publicly reveal what you’ve been working on because creators, with the exception of guys like Aaron Sorkin, generally are already filled with self-doubt. It’s fine not to like something, but telling them that their work sucks or is stupid is generally redundant: they already fear that.
Your job is to tell them what you liked, what you didn’t like, and how they could have made the thing better, in a positive, useful way. And nobody expects you to write a book; a few sentences of feedback are something every creator craves.
Did you understand the idea, and what did you think of it?
Would you watch an episode if it were produced?
What did you think of the characters, the acting, and the production?
What could the creator do to make the idea more compelling?
Where would you like to see the show go?