Larry Stark posts an essay about writing reviews of plays.
He includes, for reviewers, some instructions in theatre etiquette:
"And then, if you haven't already, learn to laugh.
That's another thing plays try to make you do, and performers expect, or at least hope for, laughter to let them know they're doing it right. I don't mean laugh emptily, but laugh when you see something is funny --- respond, naturally but honestly, don't sit there like an iceberg. Don't let the performers think you're going to come backstage later to say 'You were so funny I could hardly keep from smiling!'"
He also includes some tips on writing:
"A review should find most of its facts in the play on the stage, and things like previous plays or performances by these same people may be interesting, but they do take up space that could be better used. Such things should be mentioned if they are unavoidably relevant, but they're just distractions otherwise.
In that regard, beware The Press Packet!"
And some veiled suggestions for us theatre practitioners:
"There's an interesting quirk lurking in the cast-lists of a lot of plays: characters are named, but their functions aren't.....For the purposes of plot-summary, the character's Function is much more important than the name. And think what a nightmare it is sorting out all the work of specific soloists in A Chorus Line when all you're given in the program are twenty First Names!
I never said reviewing would be easy, did I?"
And he gives some very straightforward advice for aspiring reviewers:
"But I will say this: whatever you want to write about plays, it's a good idea to start by learning how to write straight, no-bullshit reviews of what you see --- almost as a craft. I learned that craft from a very gifted editor named Joe Hanlon, and I still treasure conversations in which we'd argue for half an hour over the wording of a single sentence. His point was that he himself hadn't seen the show, so everything I wrote had to be clear to him."
It is really worth reading in full. And Larry's Theatermirror is the largest on-line regional theatre website of which I know. So check it out.