At the beginning of the play, George and Nick seem totally different. George is old, apathetic, and washed-up. Nick is young, ambitious, and has bright prospects. By the end of the play, however, Nick is revealed as being more like George than we previously thought. He shows himself to be capable of the same sorts of vicious attacks. Also, his marriage is revealed as having similar flaws to George's.
Martha and Honey are for the most part complete opposites. Martha is cruel; Honey is kind. Martha is bitter; Honey is optimistic. Martha is quick witted; Honey is slow on the uptake. The one thing that seems to bring the two women together is their want of child. Both women desire this so much they've created imaginary offspring – Honey with her hysterical pregnancy, and Martha with her fictional son.