What were Peter Tchaikovsky's greatest accomplishments?

Since Tchaikovsky's primary accomplishment was the writing of a great deal of music, the greatest accomplishments would be a matter of taste and opinion. He is best known for works like The Nutcracker, specifically the suite from that Ballet, the 1812 Overture, the First Piano Concerto, and the Romeo and Juliet Fantasy Overture. Many of his other works are also popular, and hold firm places in the standard repertory.