After reading stories over the years from transgender women, one recurring theme that often comes up is someone in that person's life, usually her mother, telling her that she looks or acts very feminine. This post by Stana from Femulate is a good example of what I'm talking about - My Story - Part 1 . For many transgender women, this provided some of the first confirmation of how they felt inside.
I never received any positive reinforcement like that. In fact, I received the exact opposite; I was constantly told by others that I could never look like a woman, therefore I could never be a woman. I remember being in the 7th grade and having an idea for a class project that involved interviewing a historical figure. As I imagined it, I would be dressed in a woman's suit as a Barbara Walters-style reporter interviewing my classmate (I even wanted to call the presentation "40/40"). When I mentioned this to my mom, she took me to my grandmother's house and let me try on an old dress of hers, the only one she thought would fit me. When I came out in the dress, all she and my grandmother could do was laugh. Their reaction told me that I didn't look good as/like a girl so I didn't even bother arguing with them.
I received more explicit comments in this vein when I was in college. I was sent by my parents to see a Christian psychologist and even though he clearly didn't agree with being transgender, he took what could be described as a pragmatic approach in trying to convince me by saying that "it didn't matter if I had a sex change and became a woman, people would always see me as a man because my features are too masculine." He even said that he told transman he was seeing at the time basically the same thing, telling him that "his eyes were too pretty and feminine" to ever pass as a man.
This kind of feedback led me to not pursue transitioning for a long period of time. For many years, I didn't even crossdress at all and when I did, it was mainly alone in my house with the occasional trips to an anime convention.
I had always viewed this as a personal issue, my own private response to how people talked to me and treated me. But recently I realized that this kind of talk is part of maintaining what Gayle Rubin terms the "sex/gender system" in her article "The Traffic in Women: Notes on the ‘Political Economy’ of Sex." To admit that people born male can be women and that people born female can be men would completely destroy the system and physical appearance is the easiest place to attack because it's the most visible and it's also something that many trans people are uncertain and nervous about themselves. If this kind of talk prevents any trans person from being his- or herself, then the system has been maintained.
Update: For those who are interested, here is a link to Rubin's article on Google Books: "The Traffic in Women: Notes on the 'Political Economy' of Sex."