What part of our current condition can we change or make better?
How about any? I see this as a good measure of transhumanism: how much of what we are can we change?
Right now, we can alter a few percent: prosthesis, eye correction, hearing aids, pace makers, vaccines, these are body mods. We can also throw chemicals into our brains to alter perception and behavior, to a certain extent. That's a good start.
To understand what we can change, we have to understand what we are; what makes us human. This point was brought up several times: how can we tell if we ever stop being human? Physically, it's very clear; the genes, the body, the brain, the way we move and eat and mate, birth and death. Mentally, it's not so clear; but there is much research into that, and research reveals that the human mind is a product of evolution, just as much as our physical forms (no surprise there). Our behavior, desires, preferences, what makes us happy, much of it is coded by thousands of generations of adaptation. These things make us human, but they also limit us - we are stuck in this body, stuck in this mindset. We are forced to live with the default settings we got when we were born.
But there's a part of us, a small but all-important part, that can choose - our will. And we can choose to change ourselves. I think that's transhumanism, and it's fundamentally new. We have spent 10 thousand years changing our external environment to suit us. Now we turn that inward.
To quantify it, I'll use a computer analogy. Let's say I got a factory made PC that comes pre-installed with a set of hardware and software. If I were naive I would just use it out of the box. If I understand a little more, I can some RAM upgrades. A bigger hard drive. Nicer LCD screen. I can get new software - Firefox browser, OpenOffice, etc. Now it's a lot different from the factory machine. If I'm really good I'd wipe the OS and install Linux. Swap out the CPU for a quad-core. Build a custom case-mod with liquid cooling. Now the machine is not even recognizable from the one I bought. When we can change everything, we'll be post-human.
Every change is a choice. I can always choose the default and stay human. But I want to be able to make those decisions instead of being shackled with it. Only by breaking those ancient, evolutionary chains can we truly have free-will.