Well, hello there! So you got past the awful title, excellent!

I'll make this nice and quick: I reject the notion that only unicorns and alicorns can use magic in Equestria. I know the arguments about Earth Ponies being linked to nature, and Pegasi can fly and walk on clouds, very good! But I wish to build on that.

My approach here is something of a mixture of Dungeons and Dragons, and Harry Potter: Some folks (pony and non-pony) have magical potential, but would need some kind of magical object in order to channel their magical energy into casting spells: This object can be literally anything; from a wand, to a staff, to jewelry or articles of clothing. However, the issue is that it would take a long time of study, even for the most powerful individual, to be able to match the abilities of a trained unicorn: A unicorn is essentially a pony, a pony with the world's best wand not only growing out of their forehead, but wired directly into their nervous system. Most adult unicorns can cast telekinetic spells without even so much as thinking about it, but a pony using a wand or something of that ilk would have to put a lot more effort into concentrating on the object, then actually moving to make the object move: Perhaps in time it'll become as thoughtless as it is for a unicorn, but they will always need to actually use a tool, while a unicorn can just DO it. Still, with a lot of study and magical strength, a non-unicorn magic user could become an extremely powerful mage, able to wrestle with the best of the unicorns, with one except: It's far easier to take a wand out of a person's hand, than it is to pull a unicorn's horn out of its skull. If a non-unicorn magic user loses their magic channelling object, they can't use magic anymore, no matter how powerful they are.

Well, that's just a little headcanon theory I'm sharing for you guys. I'm using it in a story series I plan to write, though I lay the seeds for that in this story here:
http://www.fimfiction.net/story/327714/little-steppes-in-the-magic-shoes