The Keys of Harmony · 8:40pm May 22nd, 2016
In the season four finale, six objects became keys to the seed-chest of the Tree of Harmony, which yielded the Rainbow of Light and defeated Lord Tirek. He had gained his powers by stealing the inherent magics of the ponies of Equestria, which included their Cutie Marks.
Cutie Marks represent the unique gifts and talents of ponies, their destinies and purposes. I presume that the raw potential of a pony is crystallized when they become Marked, that their life's purpose and magic becomes channeled into a specific tendency (and the ancillary abilities which support it).
The key objects were each gained during events in which the former bearers of the Elements found it hard to embody their Elements, but did so because it was right. In each case, they found that betraying their Element might result in short-term gain, but chose otherwise to be true to themselves. What I haven't seen anyone else explain yet is that, in each case, it was hard because they had to make a choice that favored a relationship above the abilities granted by their Cutie Marks!
Applejack chose to tell the truth about her false advertisement, at the cost of fulfilling her Mark by helping the family farm get short-term publicity from Flim and Flam's miracle tonic.
Fluttershy chose to be kind by pushing the Breezies away, at the cost of fulfilling her Mark by "helping" tiny creatures only she could talk to.
Rarity chose to be generous to her friends and a former competitor, at the cost of fulfilling her Mark by focusing only on furthering her career.
Rainbow Dash chose to be loyal to her Ponyville friends, at the cost of fulfilling her Mark by winning the competition with the Wonderbolts.
Pinkie Pie chose to help Rainbow Dash laugh, at the cost of fulfilling her Mark by proving herself a better party pony than Cheese Sandwich.
Twilight Sparkle chose to free her friends, even Discord, at the cost of fulfilling her Mark by remaining in charge of the mightiest collection of magics the world had yet seen.
A friend once asked me what the difference was between personality and character. These episodes have explained it better than I ever could, with Cutie Marks exploring each pony's personality, their tendencies in everyday circumstances, and Elements being character traits that come into play when a value choice must be made.
That's a nice take of the issue, and an interesting angle on the Key of Harmony episodes!