Great discussion on this. I remember reading a fascinating article about "Equal temperament" and "Just Intonation". Noel really explained that nicely.
Years ago, when I played a lot of trumpet, I would write songs/arrangements in "standard" band keys. These keys would make playing easier on trumpets, bones, saxes, etc...
Now that I'm a vocalist/guitar player, I rarely think about what key I write in. Whatever key fits me and my wife's vocal range is what I shoot for. Even then, sometimes going up or down a half-step can make a big difference in how a song feels.