I just had an epiphany. Finally I am able to put this into words.
Things happen for a reason, people are influenced greatly by their surroundings. A chain of events caused Luna to become Nightmare Moon, and the forces of love and empathy weren't powerful enough back then to stop it from happening, and neither could it be defused.
During "Luna Eclipsed", you could notice from her manners that things were different back then.
A thousand years later she returned, and her return coincided with a most fortunate chain of events that put the Elements of Harmony to a very different use ... actual harmony. Oh my, Celestia might have put discrace on those magical sacraments in the past, by creating a great and painful division. This becomes clear when you consider that Luna didn't return as Luna, but as the same she was when she was banished.
So it was actually Equestria that had to change, that needed a thousand years to grow into a state of being able to mend that old wound. Luna had the role of the somewhat involuntary teacher (basically a nemesis), and Celestia seems at least to some degree unaware of how she should have shown more humility upon their re-uniting. That fateful act of banishing her beloved sister surely influenced her rule deeply in the following millennium, allowing her to turn Equestria into a place where love and compassion has grown strong enough to be able to right the shortcomings of the past.
Luna's strong yin energy facilitated this passive approach. We can fight certain difficulties, but at some point, everypony is at their limit, and when Luna reached that limit, she allowed herself to succumb to weakness, staying true to herself by making a great but necessary sacrifice in order to teach a world the hard way; a world where there was no feasible alternative for its betterment.
You know that wherever the sun shines, the night retreats effortlessly ... and it has to. This so easily invites exploitation.
You think Nightmare Moon was the aggressive, offensive pole in this conflict? Wasn't she to some degree a mere reflection of Celestia's approach of that nature? Her active, burning sun within - attacking problems upfront? Might she have neglected to offer Luna the space she needed for fulfillment? Did she become too full of herself without noticing and thus influenced pony society towards neglecting the night? ... Life tends to show you on the outside what is deep within yourself that you don't want to see.
Nightmare Moon was a corruption; the result of imposing the energy of the sun onto the silent beauty of the night.
This mistake shall never be repeated.
Thus, I am writing these words
as an homage to Luna's elusive greatness.
UPDATE 2015-09-17: And this might be why there's that odd contradiction in the sisters' theme and personality: Celestia likes to rule by not ruling, to just support and nourish and instill confidence and let ponies figure things out within their own potential, while Luna is surprisingly forward and active. This might have been different long time ago, with Luna being very yin and Celestia being very yang and that having been the cause for the crisis, so the sisters have now moved closer to each other and that works.