This is the criteria I usually use in terms of plot:
1/8: Garbage. I don't believe you wrote this in good faith. I don't think I've used this rating more than once or twice.
2/8: Garbage, but an attempt was made, at least.
3/8: Technically coherent, but I didn't really enjoy myself while reading it, and doubt anyone else would either.
4/8: Coherently written, but it wasn't entertaining and didn't seem to have a point.
5/8: You accomplished your objectives and created a coherent, entertaining story. However, nothing about this really stuck with me, the story was merely functional. I'm not convinced you were passionate about the idea.
6/8: Well-written, with interesting characters and strong wordcraft.
7/8: Very well-written. I would consider paying money for this. Your goals were ambitious, and you successfully used wordcraft, plot, and character to accomplish them in a meaningful way.
8/8: Your goals were ambitious, and you showed extreme writing craft in how you accomplished them. Professionally publishable. I'll be thinking about this story later on.
If there are serious typos or grammar errors, I'll usually knock my rating down several points. If the wordcraft is grammatically correct, but otherwise very dull or style-less, I'll knock it down a point or two. Exceptional wordcraft will raise my rating.
Unfinished stories, or those with major loose ends, I'll also rate lower. False choices (ie, a linear story pretending to be a cyoa) also bring my rating down.
If someone has an ambitious story that fails to accomplish its goals, I'll sometimes rate it higher out of respect for the idea. But this is unusual.