Earthpony magic is also seen in the Hearth's Warming Eve flashbacks, as the Unicorns and Pegasai seemed physically unable to grow food, hinting that only the special magic of the earth ponies allows it.
If memory serves, however, Tirek targeted Unicorns first. This could be just because most of Canterlot's population is unicorns, but it was implied that they just have their magic nearer the serfice (and so more ripe for stealing) and only after he grew powerful was he "even able to steal the magic of earth ponies and pegasai" (or something like that).
So I caught up to the latest episodes of ponies, and holy crap is A Royal Problem one of the best episodes, period. Not just this season - BEST EVER. The way it humanizes - equinizes? - the princesses is just priceless; I love seeing Celestia with a bit of an edge to her despite trying to be nice, both of them dismissing the other's tasks as slightly easier than theirs, and generally acting like people instead of hiding it behind a curtain was amazing. Twilight's freaking out showed why an outsider was needed. And man, EVIL CELESTIA?! Epic!
I did quite like this one, since we so seldom get to see life in the palace, and the idea of a reunion of two sisters after a thousand year absence being anything but rocky seems far fetched at best.
On reflection, though, I did wonder: who did Luna's job for the intervening thousand years? Celestia seems to have no understanding of it, so not her. Seems like she just raised the moon and then went to bed. So, if everypony grew up just taking dreams as they came, wouldn't Luna's return seem a bit funky, especially as she was more a creature of nightmare than a savior? Maybe the younger ponies could adapt more easily, but the adults? So really, there's about a millennium of evidence that if Luna wanted to take a night off, it wouldn't be the end of the world. Though since we often see her during the day, perhaps she does when she needs, and just works hard the rest of the time as penance for her Nightmare Moon days (even though there was a whole episode about her forgiving herself).