Express your view honestly, and explain why you feel the way you do about both the flaws and attributes. Try to hit the basics because those are the things that other players will be looking for. Above all be fair, you can be harsh, but make sure that you are fair.
Actually one of the better tutorial's I've seen was
Liberty's. She was trying to point out 300 words isn't a hard number to reach, but she ended up creating a very good primer for a budding reviewer. Ask yourself the questions she poses, it's a great guide. I use it still and I've got 100 reviews under my belt here, no one knows it all, and all of us can improve and grow.
Wrap your head around the basics first and then the more in-depth and interesting stuff can come later. The reviews I actually like reading and get the most out of as a potential player are the ones where the reviewer lets me see through their eyes for a little while, which is why I enjoy Addit's and Soli's reviews so much (and Marrend's stream of conciousness stuff is a treasure trove), but they all started with the basics.
Game making progressively gets better, so does reviewing. No-one starts as an expert.
Though a word of creative criticism of my own, Soli watches spelling and grammar pretty well and you've got three errors in this status post title so before you submit anything, run it through a spell checker. I'd recommend writing it out first in a word processing programme and then cut and paste it in. I remember him stating once he rejects about half of the reviews submitted for not meeting language standards.