Part 1: The Transhumanism Movement.
Part 2: Society is more delicate than transhumanists think.
This short essay doesn't delve into my personal ethics as applied to enhancement-- which, I must admit, I don't have figured out yet. And I'm assuming, for the sake of this essay, that a technological 'Singularity' is a feasible outcome of our current technological trajectory, or at least that significant augmentative technologies will become available in the not-too-distant future. This is a purely practical critique of full-speed-head-and-damn-the-torpedoes Transhumanism.
Transhumanism is about using technology to transcend one's humanity and becoming qualitatively more than what one was born. There are, of course, social downsides to allowing people to do this.
The Equality Objection to Transhumanism: breaking the bonds of common humanity is a serious thing.
First, let's talk about equality. In the Western public sphere, we tend to treat everyone (save children and mental patients) as exactly identical. Now, the sharp-eyed among you will notice this often doesn't make a lot of sense-- but the current excesses and irrationality involved in treating everyone as exactly identical in the public sphere are much less harmful than the excesses and necessary oversimplifications involved in treating everyone differently with respect to the the perceived value of their capabilities and potentials. More than just as a matter of efficiency or polite fiction, there’s real, generative value in the philosophy of equality, even if it doesn't completely fit reality at the seams. And I think this broad-sense every-human-is-equal liberalism we’ve built into our culture is really the only buffer we have against really nasty, heartless states of affairs that could arise from the misuse of cognitive enhancement. But I suspect that the very presence of cognitive enhancement may very well erode its own mitigating buffer.
If we look inward, it’s not a large stretch to say we’re a culture worth saving and amplifying in large part because of the liberalism and philosophy of equal worth we’ve deliberately nurtured and woven into our collective self-identity. Insofar as cognitive enhancement increases the cognitive divides within society, people will notice and it’ll put an unavoidable culture-wide strain on this philosophical outlook. We’ve spent hundreds- perhaps thousands- of years building, affirming, and lauding our bonds of common humanity and equality, and it’s now Western society’s nominal organizing principle and the glue that holds us together. Technology that threatens to rip this integral part of our social fabric apart is not progress. Or if it is, it must pay (preferably in advance) for the damage it will cause.
Transhumanists see themselves as the “good guys” (and gals… though mostly guys). Given all the good things these technologies can do, I understand why. But I’m not quite ready to grant unconditional “good guy” status as I think there are several stands of naivety that often surface in transhumanist culture, and true 'good guys' can't be naive. In this context, I think transhumanists need to acknowledge 1. the value of our carefully and painstakingly created framework of equality, 2. that transhumanism does indeed violate it, and 3. that this violation of our current social contract is an extremely serious, dangerous thing. And it’s asking a lot, but if transhumanists are going to be at the forefront of dismantling the basis for this social philosophy, I'd prefer that they offer an alternative that people can buy into that has a more nuanced understanding of human identity and human worth in this upcoming age of increasing divides. Ideally something that provides on average as much social cohesion, philosophical coherence, and spiritual nourishment as this philosophy of equality. Because if we mortgage the social bonds of the present in service of the future, that future is likely to fall apart.
I think transhumanists (and people in general) tend to think of society as a somewhat dysfunctional but intrinsically resilient entity. That, for all its warts, modern society has a solid foundation we can depend on while bootstrapping ourselves into a better mode of existence. But I think when transhumanists start to tinker with human nature, we can no longer take this for granted. After all, if you’re undermining society’s organizing principle, even with the best intentions you may break important things and deeply anger many, many people. And can you blame them? For all you’re offering, you’re also dismantling the basis for their belief and identity systems, and at least apparently pushing a system that runs counter to many of our crusading social heroes such as Martin Luther King, Jr.
I understand the promise of transhumanism: literally, to eliminate all suffering. This is not to be minimized. But I think it’s an open question whether moving to a transhuman society will break society in the process.
Of course, it's unfair to put all of this on the shoulders of transhumanism. The debate of whether we should allow these technologies into society is a probably hollow one: they have so many physical and philosophical beachheads already, and we are such an open, self-directed society that lives and breathes the ideas of potential and progress, that of course they’ll become part of society. Similarly transhumanism, in its better and more public corners, is a movement that very sincerely means well, and it's less causing the development of these transformative technologies so much as being a cheerleader for their positive uses. And asking the more realistic question of, given that this will happen, how do we make this happen in the best possible way?
But my advice to transhumanists is, do understand that society is a much more fragile thing than you probably realize, and that large parts of society may not greet you as saviors.
 Arguable, of course. Francis Fukuyama has suggested that enhancement technologies would cause us to "no longer have the characteristics that give us human dignity."
 This normative force for equality within society does have its ugly side, e.g., slowing the bright kids down for "No Child Left Behind".
 I think it's fairly clear that transhumanist technologies will increase the divides within society and corrode our culture of equality: not only will there likely be uneven access to these technologies, and uneven knowledge about them, but there's a strong status quo bias in the human psyche. These technologies will be new, different, and sometimes very strange. A divide is a divide- and causes problems- regardless of whether it happens by chance or by choice.
 The philosophy of transhumanism, though relatively developed and fleshed out, is not the presumptive solution here because it simply hasn't proven acceptable to the general public. Maybe version 2.0, 3.0, or 4.0 will be the one that finally gains traction and appeals to more than a small subset of the population. But- no offense meant to transhumanists- it's clearly not there yet.