Society's double standard for women
Men in the novel survive scandal. Anna doesn't. Tolstoy shows how 19th-century Russian aristocratic society enforces its rules almost entirely through what it does to women — exclusion, gossip, and legal disadvantage.
Passion versus duty
Anna chooses passion and loses everything. Levin chooses duty and finds something like peace. The novel doesn't say passion is wrong, but it shows what happens when passion has no structure around it.
The search for meaning
Levin spends the whole novel asking what life is for. He tries farming, philosophy, and marriage before a conversation with a peasant gives him a temporary answer. The question never fully closes, and Tolstoy treats that honestly.
Marriage and its costs
Every marriage in the novel is under pressure. Oblonsky cheats. Karenin is emotionally absent. Levin and Kitty fight constantly early on. Tolstoy doesn't romanticize marriage — he shows it as difficult work that either holds or breaks.
Fate and inevitability
The train appears at the beginning and the end. Anna herself calls the first death she witnesses an omen. Tolstoy builds a sense that certain paths, once started, are very hard to leave. Whether that's fate or psychology is a question the novel leaves open.