I am almost certain you do not need an introduction or conclusion. Forgive me if I'm wrong, it might be a requirement on another exam board, but from what my English teacher has taught us, intros and conclusions are a waste of time. If you wanted to include the point about racism in the 1930s and 2010, that would fit just as well into the paragraph about Crooks. If you really think it's necessary to write an introduction, I'd leave a couple of lines at the beginning and come back to it at the end - the core development of your essay is the most important thing.
Also, make sure you reference the author's name at least once in your essay, preferably once in each paragraph. Even though you've made differentiation between real life and fiction (by the use of 'character' and such) this makes it extra clear. It's also important because for a high grade answer you need to talk about the author's intent and effect - which is pretty difficult to do without mentioning his name. For example, when you said:
Moreover, Curley's wife is described as having 'red fingernails'
you could easily reference Steinbeck:
Moreover, Steinbeck describes Curley's wife as having 'red fingernails'
In that way, you have instantly made the reader aware that when you follow with the effect of that technique, it is a deliberate device Steinbeck used, and not just your speculation on the phrase.
Finally, just a couple of small points:
- instead of saying 'hints towards' ('This hints towards an unhappy ending') say 'foreshadows' - it's a technical term and a technique which will gain you marks
- try to support your points with quotes. You go through the whole of the paragraph about Crooks without quoting; perhaps find a quote to support your point about Crooks being racially discriminated against. It makes your argument stronger and it will help towards your grade
- as boring and redundant as it sounds, try and make explicit reference to technique. Your interpretation and description of effect is fantastic, but a well structured point is based on effect created by language, form or structure; there's a really good post about it here:
e.g. cyclical narrative, aural imagery, motifs...
I can see how it may have been more difficult to include these in this essay theme, but still try. For example, you could use the point about Curley's wife's nails and mention 'colour imagery'.
- 'red' is a word, or colour, not a phrase.
I'm sorry if this seemed really critical, it's not meant to be; you've got the beginnings of a really good essay. I hope my advice wasn't really misguided - if it's completely against what your teacher told you, ignore me; but I'm pretty sure these points compliment the mark scheme of most exam boards. Just out of interest, what exam board are you on? Is it AQA? I have that English Lit exam on the same day.
Hope I was some help. Good luck with your exam!