Failed ambition
Almost every character wants to be more than they are. Konstantin wants to revolutionize theater. Nina wants to be a great actress. Masha wants Konstantin. None of them get there. The play shows ambition as something that mostly just hurts people.
The cost of artistic life
Being an artist in this play means being self-absorbed, exploitative, or broken. Trigorin turns people into material. Arkadina protects her career over her son. Konstantin can't separate his writing from his emotional wounds. Art doesn't redeem anyone here.
Unrequited love as a chain
The play runs on a chain of one-sided desire. Medvedenko loves Masha. Masha loves Konstantin. Konstantin loves Nina. Nina loves Trigorin. Trigorin loves Arkadina, sort of. Nobody loves the person who loves them. This structure isn't comic. It's suffocating.
Careless destruction
Most of the damage in the play comes from people not thinking about consequences. Trigorin pursues Nina without considering what happens after. Arkadina dismisses Konstantin without considering what it costs him. Chekhov shows how ordinary self-absorption can ruin lives.
Time and stagnation
The two-year gap between Act Three and Act Four shows how little changes for most characters. Konstantin is still stuck. Masha is still unhappy. The estate is still decaying. Time passes but doesn't heal or improve anything.