Hi Suzi,
This is how I answered your question for myself about 3 years ago:
When I think feminine/female the associated words that come to my mind most spontaneously are: beauty, gentleness, delicacy, finesse, empathy, attention to others and no doubt seduction and sensuality also... It is both a physical image and a way of being that seem very coherent to me. I also willingly admit that it is, without any doubt, a highly idealized and "binary" image of woman. I have had enough experience of real women in my life to know that appearances can be deceptive ...
However, if I didn't have this idealized image, it is very unlikely that I would be attempting such an "unreasonable", hugely disruptive, transition, which contains so much intrinsic potential to end badly. Also, at a very practical level, for me this means that a person, represented by his/her physique, is inseparable from their attitudes and behaviors, the two being complementary. For example, it is all the more impossible for me to imagine myself living as a woman with a resolutely masculine physique that I believe our physique has a huge impact on the way we interact with others. If I am 6'6" and weigh more than 200 lbs, my impact on others will be very different from the impact I would have if I measured 5' 3" and weighed 110 lbs. Also if I have a face with gently rounded features, I will have a different impact than a person with more angular and rougher features.
In my own case, at the physical level, it is definitely the face that counts most: the softness and finesse of female features, the openness of the eyes, the beauty of women's smiles. To this day, when I look at woman around me, I am admiring, fascinated and, perhaps most of all, envious.
Without having too many illusions about what is feasible, it is probably because of this that last summer (2011), after 3 years of questioning and hesitation , I decided to do FFS (forehead, eye contour, nose and lips...) and that I will finish the work this coming summer. Even if I still have to wait before I can transition completely, I already have a face that is better aligned with how I feel deep down inside me. Not easy at all to erase 54 years of testosterone and living as a man but neither is it impossible and, overall, I am very happy with what has been done so far. Also, with a face that is already gentler than before, I feel gentler inside.
For me, all of that is still valid today and from a practical point of view, I'm also very much on the same page as Hikari, I want to be seen unquestionably as a woman by others. It just makes life so much simpler and more comfortable. Happily, after a lot of "rework", today it is mostly the case.