People are so unafraid to create and share these works: write songs, sing them, record them, and put them up on Youtube etc. to share them with everyone, and I love that. Quite a few of the songs that get shared this way aren't as "polished" as mainstream works might be; the mastering's not flawless, the takes aren't perfect, fan voice actors don't match the characters to the last degree, and indeed don't always have trained singing voices anyway. But the music is honest and sincere in a way that professionally-produced mainstream music usually isn't; not hyperreal but real.
I was thinking earlier that quite a bit of it I wouldn't share with others (outside of my friends, anyway) because I'd be concerned that they'd reject it for not living up their standards, but I'm not realizing that the problem lies with these others, in fact our modern societies in general, not bronies or their music. We've learned to expect this flashiness and glitziness, and with music in particular we've lost our ability to appreciate the simple things, the unpretentious things, the things that simply are what they are: the real things.
Compare eating a simple home-cooked meal with friends to a perfectly-choreographed dinner at a fancy yet ultimately sterile restaurant. Which would you prefer? I know I'd much rather be sitting around a campfire, sharing a big pot of chili, laughing and talking and being with the people I care deeply about.
Brony music is like that.
The brony community is sometimes said to be part of the "new sincerity" phenomenon, and while I'm usually loathe to label phenomena while they're still ongoing – it's only hindsight that's 20/20, after all –, I agree with this assessment: sincerity is one of the brony community's defining core characteristics. And I think this is particularly evident in the music. Our music may not be professional, but it's heartfelt. We do what we love, and it shows.
(On a side note — "new sincerity" is interesting anyway. A few years ago, some German women's magazines – which, despite all societal changes, are still largely about fashion, make-up and pop psychology – announced that they'd start abandoning professional models and instead use "normal women" to model clothes and accessories, in order to cease projecting unrealistic body images and beauty ideals. It's the same principle as with professionalism in music: too much of a good thing isn't good, and people are getting fed up with it, yearning for the real instead of the hyperreal.)
But music is special because we're so used to judging it harshly. Ask yourself: if someone, a fan, drew a picture of whatever they were a fan of, would you compare this picture to the works of professional artists, and judge it poorly if it didn't reach the same standard of technical excellency? Hardly; you'd appreciate it for what it is, a sincere expression of creativity and joy. Fan-created music is exactly that, too.