1. Hopes, Dreams, and Plans: George and Lennie may dream a little dream of owning a farm, but they don't get very far with their to-do list before it all crumbles in heartbreaking failure.
2. Friendship: Of Mice and Men is the equivalent of a bro hug: all sublimated emotion, gruff affection, and hearty back pats. George and Lennie don't text each other eleven times a day, and they don't like every single cat picture the other posts on Facebook—but we still get the sense that they take their friendship more seriously than anything.
3. Isolation: No man is an island… unless he's an itinerant worker during the Great Depression, and then he's about as lonely as you can get.
4. Innocence: Lennie's mental disability makes him into a child, with a child's innocence: he likes hanging out with George and petting soft things. Sounds like a great Friday night! Oh, but there's a problem: he's a child trapped in the body of a powerful man. Innocence may protect Lennie, because he never has to deal with the reality of what he's done—but it doesn't protect the people (or pets) around him.
5. Freedom and Confident: Lennie and George are tied down by their need for money. Curley's wife is limited by being a woman. Crooks is stuck because of his race. Except when they're caught up in the intensity of the dream, most characters in Of Mice and Men seem more focused on bemoaning their confinement than planning for their freedom.
Metaphors and similes are used to add depth and meaning to the poem, so it looks normal on the surface but when you re-read it you uncover a deeper meaning :)
