The biggest thing that I see that functionally defines the difference between a filker and a not-filker -- to me, at least -- is that someone who is a filker comes to play with us. Someone who is not a filker comes to play at us.
Look, I'm honest enough to admit that I have an ego and I certainly want to play my songs for the people in the circle. And there are people there whose opinion matters to me more than other people's. And there are people in the circle who I prefer to listen to more than I prefer to listen to some others.
But I go to the circle because I have more fun in the give and take and interplay than I do either giving or listening to a concert. (Please don't take this as "Bill doesn't want to do concerts". Bill is delighted to do a concert any time that he has a voice. See comment above on "hav(ing) an ego". :) A concert is a different thing from a circle and the very best filk circles are absolutely wonderful to behold. Or behear. Or something like that.)
And if I'm not having fun on a particular night, I can pick up and go do something else. I'm not trying to make my living from this, the world is not going to end if I don't get a particular song in right now (I might be frustrated, but I'm a big boy, I'll manage).
Musical tastes vary too. Some folks are going to like my stuff, others won't. And vice versa. The fact that I don't personally like something doesn't necessarily mean that it's bad, only that it doesn't suit my taste. Heck, there are folks who swear by Vegemite. :)
I know there's a point here somewhere, but I seem to have lost it.
If you find it, let me know.