Bungie was far from satisfied with the leveling system Destiny had when it was released back in September 2014. This is what Smith had to say about it:
Bungie started working on a new leveling system shortly after the original release of Destiny. The new leveling system was not applied until the Taken King because of compatibility issues with the past expansions The Dark Below and House of Wolves.
Make sure to keep your eyes and ears open. Bungie is far from done with Destiny. They have hinted that they intend to add a lot more to the popular first-person MMO shooter than a new leveling system.
The full interview between Kotaku and Smith can be found here.