For me, my ideal is the lowest fat percentage my body will support healthily. Most people I know would freak if I said that, at which point I would reiterate and emphasize the "HEALTHILY" point, which is non-negotiable for me. I just don't have the willpower to not eat and exercise all the time.
I think the OP raised an interesting point - people in the USA especially don't like hearing that thin people want to get even thinner, even if it's still healthy. I'm planning even now on how to deal with my family & friends on this - it WILL be an issue.
I'm not sure what is considered a "featherweight" here, but with me wanting to lose another 40+ lbs, I don't think I'm one.

Hope it's okay if I post here.
About the big bones thing - I still think I have a large frame. Maybe not, but at this point it's likely. I've lost about half an inch on my wrist, but it still says my frame is large. ~sigh~ I also have big ankles and feet (though not unattractive, which is nice).
And RubyGuggenheim, I'm with you guys! I'm down to 164 from 207 and barely lost 2 sizes. So frustrating... I'm hoping when I hit a lower weight I'll really start shrinking. Glad to know I'm not alone on that!