Two other trends I have found in reading other's reviews that should be, IMO, Big Don'ts:

1) Harshly judging a first chapter because it doesn't have enough of the "good stuff" (whatever you're into) - As a writer, I always try to start a story with a bang. I know that's important. But, let's try to cut the newer writers a break in this department. Effectively achieving balance between plot and character development is difficult to master. I don't know if slamming newbies with 1 and 2 star reviews is the way to get the message across. I am surprised many victims of this haven't just given up (and some have, unfortunately... look at all the one chapter stories out there). Lastly, if you feel your preliminary judgement is warrented, at least do the author the courtesy of re-reviewing the story once more content is added.

2) Laying down arbitrary rules like "a story should be 'x' pages long." Says who? Hey, the size is posted on every story link here. If it's beyond your attention span or interest level, then don't read it. Sorry for flaring, I was just thinking of a certain reviewer (who shall remain nameless).