Tools of Characterization
We're going to go out on a limb here and say that George's last name probably refers to John Milton's Paradise Lost, which includes the Biblical stories of Adam, Eve, Cain, and Abel. George and Lennie's loss of the mythical dream farm sounds to us a lot like Adam and Eve's expulsion from the Garden of Eden, while George and Lennie's relationship is almost an inversion of the story of Cain and Abel, where Cain kills his brother—but not out of mercy.
Lennie Small's name is even more straightforwardly symbolic. Lennie is obviously not "small" physically, so his name is something of a joke to the people that first meet him. But he is small in another way—he's small of mind. The contradiction between his name and his size is similar to the contradiction that we see in his behavior: he wants to be gentle, but he just ends up killing the things he loves.
And then there's the unnamed women: Curley's wife. She was no name, because she's just an object, someone else's "property." Curley's wife can only be seen in reference to her husband, who (supposedly) owns and controls her body, and by extension, her. With a name, the men might have to relate to her as a person, but there's no need for that kind of social nicety. Like the unnamed "boss," Curley's wife is only important to note for the limited role she plays as Curley's wife.
Speech and Dialogue
The language of the characters is meant to represent the way normal, working class ranch people of Depression era America would speak. Most vocabulary is of an everyday kind, except for words particular to farm equipment and jobs (like skinners, swampers, and buckers). In the dialogue, Steinbeck uses slang, vulgarities, and non-standard terms ("ain't," "would of," "brang," and so on) to convey an authentic sense of the characters. The idiosyncratic speech patterns, obscenities, and casual lingo ("she's a loo loo," "Curley's got ants in his pants") help recreate a particular time, place, and social strata that make the book sound real.
It's interesting to note that everyone in the book seems to use rather similar language. The boss doesn't speak with any more refinement than the ranch guys. Lennie, though he's slow, isn't less able to communicate with words than others. Even Crooks, who is constantly made separate because he's black, speaks just the same as anybody else. It's almost as if Steinbeck puts language to work as an equalizing force: as long as a man can speak for himself, his story is as important as any other.