1) People are people. In other words, there's less difference between the working class and the ruling class than you might think.
2) Sometimes mages don't even realize that they have magic to begin with -- they just know that they have a talent for gardening or dancing or something else. And when they're first told that they have magic, they don't really believe it. (There's also a case where a young mage tries to pretend that her magic doesn't exist, which is different but sort of along these lines.)
3) Sometimes magic shows itself in very ordinary things. For example there's one mage who dances his magic, and another whose magic is in working with animals.
4) Fighting for what's right (whether you're a lone knight who's traveling or law enforcement in town) isn't all glorious and exciting. It's generally dull and boring, and when things do get exciting you want the dull and boring times to come back.
These are the themes that come to mind when I think about Tamora Pierce.
Does anyone else here read Tamora Pierce? Can you think of any other major themes in her books?