Sorry for getting back to you so late on this.
So, I want to make a disclaimer right up front that I'm only speaking for myself on this. I know that seems obvious, but I just don't want to make it seem like my case is typical for most transsexuals/transgender people. I'm not even sure I fit the textbook definition of transsexual, or that that's exactly what I have or not, so I might not be the best example.
Anyway...
Ok, so the thing is, it goes beyond just gender expression (though that is certainly part of it.) It's an actual dissatisfaction with ones body. For me (and I know this is different than a lot of transsexuals' experiences) I don't feel uncomfortable in my body as much as I feel like I should be female (my body, I mean). When I see women, just about every time I see a woman, I get an overwhelming feeling of longing and sadness, because that feels like who I really am, and who I need to actually physically be. It's like I was a woman and then woke up one day, inexplicably, in a male body, and I need to get back to who I was. It's felt that way since as far back as I can remember. So this goes way beyond socially constructed gender rules (which I don't believe in, anyway; I think everyone should be able to be who they are and act and dress how they like). This goes to feeling like one is in the actual wrong physical body.
Now, the sort of textbook definition of transsexual is someone who is uncomfortable in ones own body in addition to what I described above. Like I said, I can't say I really fit that, as I don't feel uncomfortable in my own skin, but a lot of transsexuals do (one of the reasons I question if I'm really transsexual). For them, their bodies don't feel right at all. I've heard it described from people who have actually had the big surgery this way: they said they felt that after their surgery, everything felt right immediately, that it was almost like their brain was mapped to have a female body (in this example, obviously reversed for FTM) and it felt completely natural and normal afterward because their brain (or how it was wired, rather) was finally matched up with the body it was supposed to have all along. Which does go with the theory that the brain in transsexuals is wired incorrectly (female brain, male body or male brain, female body). There's evidence of this, as someone mentioned in another thread, I believe, where they're finding that there's a part of the brain that is smaller in females than in males, and they've done autopsies on male to female transsexuals and found that this part of the brain was smaller in them, i.e. that they had, essentially, a "female" brain, at least that part of the brain, anyway. Which seems to back up the consensus among the community for a long, long time, which is that transsexuals were wired for the opposite sex.
Anyway, I don't know if I explained that properly, but I hope it helped to clarify things. Please let me know if that didn't make sense or there's something else you'd like to know about.
And please correct me if I got anything else wrong there, fellow transgender peoples (God, I wish I could come up with a decent term for that.)