The problem, for my tastes, is the subject itself. Each melodic segment within the subject ends with a long note and lacks rythmic variety. The combination of those two factors is that it loses momentum and ends up sounding like something is missing even after other voices join the mix. There also isn't any real counterpoint there, just repeating rythmic layers a la hip-hop.
Where the piano plays the subject in the right hand while playing a fast accompaniment with the right hand is another problem area. It sounds a little too simple. Let the left hand do a bit more work and turn the accompaniment into it's own one-handed polyphonic showcase as, high above it, the left hand slowly plays the primary subject as Bach often does in his choral music.