What draws me to Bach, again and again, is the geometry of his work. Each piece has its own internal systems and layers, some immediately evident, some requiring more digging, and the order is elegant and inevitable. Once you have a sense of what the piece is doing, you'll know immediately when you hit a wrong note, even on the first pass. It is as precise as the spiral of a fern fiddlehead or the ice crystals in a snowflake.
For me that structure offers a massive sense of calm. It might seem restrictive at first glance—and I think a lot of Baroque music does get trapped by its own forms—but in Bach it opens to something far more expansive.
Some of this openness might come from the work of learning Bach. I focus on two or three measures at a time (a method that I recently learned was also Yo-Yo Ma's approach to Bach as a child). This is often dull, often frustrating, but it seems to be the only way I can do it, and the very dullness of the repetition is what opens the door for the life of the music to get in. As the Zen advice goes, if you're bored doing something for one minute, try it for two; if you're bored doing it for two minutes, try it for four; and so on.
Steady practice also means that my time at the piano is a reliable indicator of my headspace. I've heard that, for most instrumentalists, there's very little difference in innate physical capacity for playing fast—that is, the guys who can shred on guitar didn't just naturally luck into superhuman finger speed. Most of it comes down to practice and focus. Playing a detailed melody at tempo requires such concentration that I'm noticeably better if I spend a minute meditating beforehand. When I'm preoccupied, it becomes obvious in lags and skips. Equally dangerous is the self-congratulatory lapse, when I notice a difficult passage has gone well and then grind to a halt in the next measure. This holds some lessons for performance with the band—there's automatically an extra level of distraction when you're in front of an audience, and you don't get the luxury of meditating before every song.
Anyway, I don't know why it didn't occur to me before now to look up Bach's first drafts, but it didn't. And oh, I love them: