Medi brings up some good questions you should ask yourself when reviewing. Have you ever wondered why you loved a particular piece of writing? What made it work? What words were used? Is the dialog dynamic and real? Do you feel like you were there next to the heroine when the villian kidnapped her from her parents?
Writers don't hope for the best in writing, we work at the craft and make it better. Is piece logical? Are the rules of writing followed? If the rules are broken, does it emphasize a point and give merit?
The top boxes are there for general criticism and praise. The inline comments are gold. If a piece has a serious flaw, like characterization or setting, you mention it in one of those boxes. For specifics, the inline comments can illuminate what works and what doesn't. You don't have to be an expert, but you should try to work on being an expert. That alone will make for better writing and reviewing.

hotspot46, it's not your responsibility to rectify issues with other people's works - you're not taking over their work and re-writting it. You're giving them feedback so that they know what to change.
Does the dialog seem awkwardly or inappropriate?
Is there a plot hole?
An unexplained inconsistency or incongruity?
Are you confused by any of the sentences or actions?
You don't have to make the sentences less confusing or fix the plot holes or dialog, you just need to point out what you feel is wrong with it.
The only thing is, DO NOT just say something like the dialog is awkward in the score box. That's not useful. Use the in-line feature to point out the issue by saying things like:
this person is supposed to be a hillbilly with a thick accent, but they are speaking perfect English.
The more inline comments you do that aren't grammar or spelling, the better.