Interesting suggestions... I agree that behavior is a key observable factor, but I don't think it's accurate to suggest that human behavior (as it relates to sex and gender) is "largely arbitrary and not natural." Hormones do affect our behaviors, and while we are creatures of reason, we do still have instincts. Many observed gender-related behaviors (such as wearing three-piece suits and wingtip shoes vs. dresses, makeup, and high heels) are cultural constructs (or as you suggest, "artificial"), but most of them are based on the biological tendencies of males to be more masculine and females to be more feminine.

Were we all bald and wore the same outfit (in the style of THX 1138, for example), and didn't have the medical technology to provide transsexual hormone treatments or surgeries, transsexuals would still exist. Sex-specific behaviors and social gender roles would still exist, and homosexuals, intersexuals, and transsexuals (as well as those who consider themselves androgynous, neuter, genderqueer, etc.) would not fit the typical patterns for male and female behaviors.

Granted, there will always be those who are "inclined to behave outside of the socially defined expectation," as you put it, but transsexualism isn't simply about the presence of gender-based social norms and the willingness to break or redefine them. It's about identity, and it's primarily internal. As long as there is sex differentiation (male and female) within our species, there will always be natural gender differentiation (masculine and feminine). The specific ways in which those gender differences are expressed might vary from culture to culture (mannerisms, vocabulary, dress, social status, etc.), but the chromosomal and hormonal differences between the sexes dictate that there will be some natural behavioral differences between the two sexes. Even among other animals, I wouldn't consider those differences mere "behavioral quirks," but essential differences that complement each other.

In my opinion, the only social changes that transgenderists might bring about are acceptance and tolerance of those who don't fit the prescribed social norms (or "artificially enhanced behaviors") by those who do. The prescribed norms have changed over time, and from place to place, but I don't think the changes have ever been tranny-driven, if you'll allow me that phrase. ;)

RE: Criticism of a Gender Theory, and a Scientist Under Siege