For a symphony written by a teenager, it is extremely difficult to do well, and extremely powerful when it is. The scherzo could very easily be danced to as a waltz at court, while the final movement in general, and the last three minutes in particular, is a powerful tribute to both struggle and breakthrough victory--some day people will stop using Wagner's Ride of the Valkyries in movies for the gazillionth time and set something to the end of Mahler's First. It takes an hour to listen to, and worth the hour to hear: go here.
Not a teenager. Mahler was 27 in 1888.