I'll add something about vocals. Funny that you could probably get 200 posts about bluegrass and there be next to no comment on vocals.
The same discipline that applies to the instrumental prowess of bluegrass musicians apply to the vocals. Vocals in bluegrass sound as tightly scripted as the instrumental breaks and choreography. The looser harmonies of an old-time band, or a non-bluegrass gospel group may not be music to the ears of a bluegrass musician because as Martin says, discipline is so important to the music. a get together of folk musicians can surely drive a bluegrass musician nuts because it can be pretty free-wheeling and loosey-goosey.

I think you hit on the key, Martin. Discipline. Now you step into the realm of personal taste. Is discipline inherently superior to spontaniety? Or is spontaneity superior to discipline. I don't think there is a "superior" involved. It depends on what you like... with tons of exceptions. There are some bluegrass vocals that are tighter than a drum that I really like, because they are so meticuously realized. Other vocals in bluegrass I find too predictable, because there seems to be such a strong (rigid?, traditional?) structure for bluegrass vocals that they lose all individuality in my mind. I've heard old-time bands that were just plain sloppy, and some who were having a good time and taking chances, where the harmonies and phrasing weren't precision, but full of spirit.

I don't think that bluegrass is better or inferior to old-time or folk music. Just different The rare thing to me is to meet people who truly enjoy (not just SAY they enjoy) both forms of music. When people say they like bluegrass AND folk music And old-time music, the criticisms of one or the other types of music usually seeps out. "I like bluegrass and folk music, but folk music doesn't have any energy or edge." Or, I like bluegrass and old-time string bands, but bluegrass music is too rigid."

When is discipline "rigid?" When does expressive and free-wheeling become just plain old sloppy or lazy?

Depends on case by case, night by night, song by song.

But, you nailed it, Martin. For me, discipline is admirable and impressive, but it doesn't get me in the gut. For you it does. Good for you! Good for me!

Jerry