This episode, like all the others, was full of cute and funny moments. However, more than any other episode, the plot didn't really make much sense. I was expecting an episode establishing the founding of Equestria to be a little more consistent with what we've been shown so far.
If the unicorns were the ones that managed the sun and moon before, at what point did the alicorns take over that responsibility? For that matter, what great purpose do the unicorns now serve?
If Equestria was founded by pegasi, unicorns and earth ponies before the alicorns, why would their flag have Celestia and Luna on it?
The only explanation I can think of for all this is that the alicorns must have conquered and taken control of Equestria some time after its founding and now rule over it with an iron hoof. The alicorns established themselves as godlike rulers and keep all other ponies beholden to them as subjects and worshippers.
Since unicorns are the only other types of ponies able to use magic, albeit nowhere near as skillfully as alicorns, they earned special status in their eyes and became more wealthy and influential than the other pony races, despite no longer taking such an important role in the balancing of the world. After all, the alicorns can't have the unicorns upset and trying to usurp them.
Well this explains why Celestia keeps bodyguards near her at all times. Somepony who rejects her tyrannical rule of inequality may attempt an assassination at any time.
The more I learn about this world the more scary and depressing it becomes.
I believe in no God, no invisible man in the sky.
-Andrew Ryan, Bioshock
...and don't forget the ponies.