When Destiny initially released, it featured a Light system dependant leveling system. It was quite complicated and annoying at best. Once players reached level 20 by gaining XP the normal way (killing enemies, completing quests, etc.) their Guardians would switch to the Light System. This system depended on gear having Light Level attached to it — basically a gear score.
The problem with this was your level was dependent on using gear with the highest Light Level, regardless of whether or not the stats were good for your Guardian or not. The latest expansion for the game, The Taken King, removed this system and replaced it with a new more straight forward system.
According to Bungie's creative director Luke Smith the developers knew that the original leveling system was an issue,
"The original Light system, when that went out the door, we were not thrilled with it," Smith said to Kotaku. "We knew when that went out the door that we were going to need to revisit it. So that work basically began in earnest probably September or October [of 2014]. We had already started to work on a new proposal, the proposal that would become the Light 2.0 that's in the game now. We were working on that as early as last fall."
Ever since Bungie released Destiny, they have been hard at work implementing changes to the leveling system through DLC – which ultimately led to the overhaul The Taken King.
"We had to look at DLC 1 [The Dark Below] and DLC 2 [House of Wolves] and say, 'OK, well, how can we set those two DLCs up to allowing for this [leveling] pivot?' So in DLC 2, that's why you see the introduction of the Etheric Light mechanic, because we were trying to get players to converge quickly at a single, common level because we knew where we were going to take them next."
