In a livestream yesterday Bungie revealed a couple of dozen of features that will be making their way into the expansion. Existing characters will get a new overall level based on their best gear, and Destiny will switch over to a traditional XP-based leveling system with a cap of 40. You will want to be level 25 to get into The Taken King content, although those who buy the expansion will get an item to bump a character to that level.
Among the new toys that players will get to play with is an improved bounty system, additional ghosts, a collection feature, purchasable faction gear, field test weapons, more vault space, more dances, and the ability to wield swords. Swords in space! Truly, we live in glorious times.