How much do you expect when someone does something for you free of charge?

I suspect this will turn into a bit more of a rant than a question, but I do actually have a question.

I make art. I draw, I paint, but I spend the most time doing graphic art. Usually if a friend has an idea or an image they’d like me to create for them, I do it without hesitation. I appreciate having new and creative ideas, and I also like the practice. Plus I just think it’s a nice thing to do.
I usually show a work in progress, to make sure that the general idea is what they had in mind. At this point most people will say either “yes, that’s great!” or “I had something more like this in mind…”
Which is perfect.
However, I am noticing lately that more and more people are becoming nitpicky over details. Things that really have no bearing over the general concept or that detract from my artistic license. Many of these pieces take me hours, and often days to finish completely. I’m not sure if maybe it is that people don’t understand what really goes into making some of these things, or if I’m completely wrong. I just can’t help but feel that if the tables were turned, and a friend was doing me a favor or a service for free, I wouldn’t treat them as though I were paying them for their work.

I guess I feel like I have less right to nitpick when I’m not actually purchasing something.
What do you think? What do you expect when you get something for free? Am I being unreasonable?

I’ll add that, on the contrary, when I am selling a piece of work.. I not only listen to the details that the buyer gives me, but I ask them if they don’t mention it.