I first ask if the story resides in the a genre I typically read. For example, even fantasy as a genre is quite varied. Often I will see a review--usually negative--from a person who obviously is unfamiliar with a particular sub-genre. So, if you are reading something very different from what you've read before I'd be cautious giving it a negative review. You may simply be doing the author a disservice while simultaneously looking stupid.
Second, I look for all the good things about the story, such as creativity. I have personally been amazed at the imagination I've seen displayed on SH. Humor is also very difficult to write and I believe should be applauded. (Especially, considering the divisions in the USA right now.) If creativity and humor are present I rarely give a review below three stars.
Things like grammar, even spelling, to me are unimportant. (Wow, I can't believe I said that.) I know grammar is important because we must understand what the author means. But to me what is most important is my emotional connection with the main character. If I have that, everything else sort of falls away.
To summarize from most to least important:
1) My emotional connection to the MC
2) Creativity and Humor
3) Mechanics such as grammar
--Kidd