I generally give them a rubric with the assignment so there's not much subjectivity about it.
You write a good introduction that's more than a couple of disjointed sentences, you get all 10 points. a worse one might be 8 points or less. you got a succinct properly-placed thesis statement, 5 points. you use at least two direct quotations supporting your thesis, 15 points. etc.
the nut-kicking kid lost points for not explaining enough and for being too informal, but he had the right idea--Scout DID show courage once by kicking a man in the nuts (when he came to lynch Tom Robinson)