Steinbeck is a good writer and Of Mice and Men is a fine enough example of that. Sometimes he's a little too descriptive for my taste but plot wise, at least in this book, he gets right to the point.
The characters weren't very developed but they were decent enough I suppose. I didn't really care for any of them at all though. I actually ended up caring more for the animals. Lennie was an idiot, George was a guy who was either very afraid to be alone or felt he had a responsibility to uphold in looking after Lennie. He complained about it either way and always said he'd be better off without Lennie (true if he felt he had a responsibility, untrue if he was afraid to be alone). Candy was a man who was looking for anything to keep himself from becoming unneeded. Curley was an asshole. Curley's Wife was a stereotypical slut. Slim seemed decent enough though in the end the author made him the only (visible) justification of George's killing of Lennie.
All the symbolism in the book gives away the ending before the book's even half way through. I predicted the ending exactly as it happened only a little after I was half way through. And I literally mean exactly. This is one of the most predictable books I have ever read. (Though tragedies tend to be predictable)
The moral, if there is one, could either be that the weak are better off dead (as implied by Candy's dog and Lennie) or that the inevitable will eventually come and sometimes you just have to make the best of it (As implied when Candy said he should have shot his dog rather than a stranger doing it. Or at the end when George shoots Lennie himself while telling him happy things instead of him getting mad and having Curley kill him. Both would have the same result, death, it just happens a bit nicer). I guess I prefer the latter moral, but I don't care much for either.
Some may say that the moral is that everyone needs a friend or something along those lines, like the negro, Crooks, in the book said. Some, as I imagine, may say that George is the real victim even more-so than Lennie since he lost his only friend and is now alone. Again, don't care much for that either.
Some even interpret it religiously, saying that in death Lennie will finally be able to tend to the rabbits in heaven. Ehh.
The dialogue is repetitive as hell. For such a short book you'll hear the words "tending to rabbits" more than enough times. All the conversations feel the same as previous ones, just rehashed. Steinbeck does get to the point as I said plot wise, but he does tend to repeat himself as if to really drill it into your head, which I find unnecessary.
I'm usually not a fan of written slang but it was done decently here and was believable.
One thing that many probably hadn't noticed about the book is that the ranch serves as a microcosm of the world, which I find interesting and well played if it had been the author's intention.
In conclusion: there are (for the most part) two types of books. Those that appeal to your intelligence and those that appeal to your emotions. I prefer the former but can also thoroughly enjoy the latter. And when combined right I think it makes for a very good book. Books that appeal to your intelligence make you think and present to you interesting ideas or look at things from a different angle. Books that appeal to emotions get you to sympathize with the characters or the story. Of Mice and Men clearly takes the emotional path for the most part. It's just too bad I didn't actually care much about the characters or the situation. (Though I'm not so cold-hearted that I don't understand it, I do, I just don't care for it.)
Should also be noted that I see a lot of people giving low ratings to this book because they found it disturbing; I wasn't disturbed so much as I was bored.
This book gets about a 1.5/5 for me, mostly due to the good writing.