I think I am pretty much mentally androgynous. I love being a girl and the many perks that come with it -- makeup, pretty clothes, high heels, skirts (so much more comfortable than pants!), and so on. I love that I can have long, long hair without anyone making assumptions that I'm a hippy or a druggie or whatever. However, my interests are pretty stereotypically masculine -- video and online gaming, football, tech, and so on.
My mom and I were talking about transgenderism (two of my good friends, one of whom is MtF and the other of whom is a natal woman, just had a daughter, which is why the subject came up) and I said, "You know, I feel really balanced, gender-wise. I'm happy to be a woman, but I think I'd be just as happy as a guy." I paused, and added, "I'd be gay as the day is long, but happy!" She cracked up (I'm bi, but I am happier with men).
I am happy with the way I am, but I can still see how someone could feel they were in the wrong body. It's not a matter of, "I'm a girl so I better do x, y, and z," as it is a matter of really feeling that you were born in a body that is all wrong. There are even some studies that show the brains of transgendered persons are different from the brain of a person who was born the right gender. I truly think gender is a fluid thing, and support the idea of a "gender spectrum." Some people are 100% female and some are 100% male, but most people have some of both in them!