You should stress, however, that you haven't read the entire work. You can't read five chapters out of a 300 chapter story and then lead readers to believe you're describing the entire story.
Things I generally try to pay attention to:
Originality - What makes this stand out from the pack? Does it adhere to usual tropes or conventions or break away? No story is truly original at its core, but at the same time, I should be able to read what I'm reading without feeling like I could see the same story elsewhere. The Smell Collector is a story with high originality marks, because it's something new and offbeat, and No Man an Island was unique enough, even if I ran into other issues that kept me from finishing it. Diary of a Runner did get a less than stellar review from me, and part of that was because it read like Walking Dead fanfiction.
Fluidity - how easy is it to get into? How well does one section connect to the rest? Does it maintain interest? Do the chapter endings feel like endings? Are there elements (grammatical, flavor, stylistic, or continuity-wise) that interrupt the flow of things? Is it amusing or does it drag? I pay a lot of attention to this in my writing and in stuff I read, and it tends to feature heavily in my reviews. I remember Interviewing Leather was the sort of work that grabbed me and swept me up into the story. At no point did I stop and think, "I should stop reading for today." I've reviewed several works, though, that slogged. Because they're works I stopped reading fairly early on, as a general rule, I can't remember them well enough to name names.
Structure - Worldbuilding, premise, key narrative elements, character. The big building blocks. Does it fit together, on a macro level? Can I sum the world, premise, story or cast up in one paragraph? In one sentence (admittedly abbreviated)? Can I grok it, on a micro level, with individual characters making sense in how they're put together? Individual setting elements? Is there verisimilitude? An internal logic?
Elegance - How's the presentation? Is it clear the author made an effort, are there obvious errors that detract from the story? A top notch story with a bland site is generally going to get high reviews from me, but a bad site with glaring visuals can take away from the story because it makes things physically painful to read. Effort goes a long way, and a lack of effort conveys a lot about the rest of the story. Caelum Lex gleams with polish. So does the now-defunct 'Life in Motion', which unfortunately lost marks in this department, because the beautiful, well-crafted site had neon green astroturf in the background, so garish it left pink afterimages in my eyes for a full minute after I looked away. The Ballad of the Emerald Bard had sloppy writing that could have been fixed with a once-over, an interface that made page loads take a full minute, and falling snow that obscured the text and distracted the eye.
Above all, though, I try to speak from the heart. I go into any story with a genuine hope that it's going to be a good read, and I feel a genuine disappointment if and when it isn't. In such a case, I try to convey to the reader why it wasn't what I hoped it might be, where it maybe hit the right marks, and where it fell short. I'm honest and I try to be upfront about everything I can. I think that's all any reviewer can expect of themselves.