Yeah that is true, Earth ponies are suppose to have a special ability with the earth or something, but I don't see how it compares to flying or unicorns. The unicorns are really over powered because if they are powerful enough they can have levitation spells and fake wings.
Unicorns aren't over-powered. Unicorns all have an affinity for a specific magic (Rarity and her ability to find gems). Twilight's affinity is for magic, which is why she can do all sorts of awesome spells (and copy Rarity's gem spell). 99.999% of unicorns are incapable of doing anything but one cool spell and some basic magic (levitate books).
As for characters, no, I don't much like Rarity either. I think hers was a persobality that Hasbro pretty much insisted on, and the writers just had to sort of work around.
Rarity really develops later. After Fluttershy she probably develops the most as a character. I think she's my favorite (along with Twilight Sparkle, Fluttershy, Rainbow Dash, Pinkie Pie, and not-Applejack
)