I'm having a hard time agreeing with you on 3, I think that each species of ponies have their own kind of magic that makes them unique to each other. For example, the Pegasus are not only gifted with their wings, but control the weather and the sky. Not even Unicorns can step on clouds without falling through. The Earth ponies I feel have a lot more then what meets the eye. Other then being the sole provider of food in Equestria, they seem like they would also be the strongest physically of the 3 species as well as live much longer (Granny Smith admitted to being alive during the founding of ponyvile, and Twilight told Spike "Ponyvile was started by Earth Ponies, so for hundreds of years they never used magic to clean up Winter". By looking at the Manehatten architecture in Cutie Mark Chronicles, it also seems like the Earth Ponies have a more modern design to their cities with lights and possibly electricity rather then the Victorian-style of Canterlot and the Pegasus' sky fortress.
Also consider that Twilight is one of the few Unicorns that spent her entire life in the study of magic as well as having her cutie mark symbolize magic itself; I cannot recall any other Unicorns preforming anything more then simple levitation.
Granted this is all headcanon for a children's show, but I the whole 'Unicorn Master Race' thing is a little one-sided when Twilight is the only Unicorn we know who can preform all these incredible feats with magic.