Wasted potential
The play is built on characters who could have been something else. Vanya could have lived his own life. Astrov could have been a reformer. Yelena could have been an artist. None of them acted in time, and now the window is closed. Chekhov doesn't blame them. He just shows the cost.
Disillusionment
Vanya's collapse begins the moment he stops believing in Serebryakov. The professor hasn't changed. Vanya's perception has. The play shows how much of a person's identity can rest on a belief that turns out to be wrong, and what's left when that belief goes.
The passage of time
Time in this play is the real antagonist. Characters are too old to start over, too young to stop caring. The estate itself is a kind of clock, measuring out seasons of labor that lead nowhere. Chekhov makes you feel the weight of years that can't be recovered.
Passivity and paralysis
Almost every character knows what is wrong and does nothing about it. Yelena stays in a loveless marriage. Vanya complains but doesn't leave. Sonya loves in silence. The one moment of action, the shooting, fails. The play asks whether these people are weak or simply human.
The gap between ideals and reality
Astrov talks about saving the forests while his own life deteriorates. Vanya believed in a great man who turned out to be ordinary. Sonya hopes for love that will never come. The play puts ideals up against the grinding reality of provincial life and shows what happens to them.