The argument "eugenics is nazi" is just a bit lame.
So far the main problem with eugenics being applied today is that a society of fashions and short term vision would prefer cultural and cosmetic change that would rather damage the human gene pool than ameliorate it. But in facts, I don't think anyone would rationally be against bettering the human race as a whole.
The benefits of eugenics, through genetic engineering are multiple. Some of them as important as "significantly reducing cancer" and "significantly reducing the occurrence of genetic diseases" and one by-product of it would be the significant reduction of the work-overload of the medical professions.
There is also the possibility that the human specie is undergoing slow, but accelerating, disgenic evolution. The way Evolution has shaped our genes in their current form is by systematically eliminating the weaker specimens of the population. Since recently (35 000 years maybe), the human specie is more and more able to allow the one that would have died in the jungle to continue living, and to spawn offspring. In a way that, since history is recorded, people with poor genetic material are just as likely to transmit their traits than those with better genetic material.
In the jungle, the bad sight that I'm suffering of would have made me the prey of the first predator that happened to track me as soon as my mother would have stopped nurturing me. Even being at the top of the food pyramid I would not have survived long, being hardly able to hunt my bread. In the world of today, I'm wearing contact lenses and this terrible genetic trait barely hinders my capacity to find sexual partners.
Certain cultural activities endemic to human specie are actually accomplishing the exact opposite of what Evolution was doing, by crushing the fittest, to the advantage of the unfit. Through traditions like "war" for example, where the physically strongest specimen of a population is eradicated, leaving alive only the weakest.
As for the intellectually strong, I'm still waiting for proof that it is in any way bound to genetics. But if it was ever proven, the state of affairs of today's civilization tends to have the dumb to reproduce more than the clever anyway.
So it seems that the evolution of human specie is halted, in the best of cases, or even degenerating the design. Though it is hard to prove, given the timespan of the event, and the poor records from the past, we cannot deny the possibility of it.
The drawbacks, I see as: distancing a future generation to former ones (not only culturally as it is now, but genetically) and the huge problem of "making the right choice".
If genetic engineering is available on the market and the tendency of the epoch is to prefer big eyes or small hands for example, we might face a whole generation of small-handed big-eyed kids, that would definitely be a step backward in a quest for a more solid human, and ironically would probably trigger a fashion for small-eyed, big-handed types :)
Another issue is the availability of the technique, that might dig another gap between the rich and the poor. Steps against this effect are seldom taken, but there are ways to smoothen the curve, and even ways to flatten it. We just need to live within a system that would automatically prefer the more ethical way (which goes a bit off-topic, read some of my (or other's) stuff on political organization).
And finally the potential malicious use of it: The mad scientist breading an army of docile highly intelligent and courageous goons to take over the whole world, muhahaha ; the whole world!
Or more realistically, the mad politician creating an army of brainless, docile citizen for the greatest joy of the market economy.
That's it for the drawbacks.
The way to eugenics is arduous, but in my sense, the reward in the end is worth every step of it.
So... where to draw the line?
As of today, the knowledge in genetic engineering is far too poor to start meddling with our own genome. Once again, helpless individuals from other species are being volunteered to serve science. But if we can accept that there will come a time when we will know our genetic material as well as we now know the solar system (which was for long forbidden to human knowledge), we need to be prepared for when it comes.
Let's not repeat the mistake of Nuclear Power, that went so quickly from discovery (Becquerel 1896), to abuse (Hiroshima, Nagasaki, 1945). Fifty years. When the time comes, the debate must have already been taking place for long enough that the people would be acquainted to it's implications.
The goals of eugenics fall in two category.
The first one would be: "willfully tweaking the human genetic pool in order to remove its defects", while the other would be: "willfully tweaking the human genetic pool in order to enhance its qualities".
On an ethical point of view, they differ in the way that the first one, removing defect, is a lot easier to justify. But there are some points of the second, enhancing the pool, that should be considered rationally.
As said above, I suffer of a bad sight, that was diagnosed when I was 4 years old. It's a recessive trait, none of my parents have it, neither my sisters. But I certainly would have much benefited for not having to deal with it my whole life.
If this case seems acceptable to you, then think of the millions of people around the world being seriously handicapped by a genetic trait. Most orphan diseases could be done with, for all time. Part of history, like the Plague and Smallpox.
And on an even more beneficial level: many cancers develop more easily because of a genetic susceptibility. Eugenics can fix that, in time.
Syndromes like that of Down or Autism are clearly of genetic origin. But many believe that they are forms of neurodiversity. I'm hereby attracting attention on the fact that a trait must be clearly, without the shadow of a doubt, identified as undesirable in order to undergo slow removal through the tools of genetic engineering. Bad sight is definitely one. Susceptibility to cancer might need a bit of research but I believe it will be deemed such as well.
And the list goes on. case by case, each trait of a human should be screened. "Is this trait much influenced by genotype?", "Is this trait a definite bummer?", "Is it possible to remove it without affecting other parts of the phenotype?", "Is it desirable for this trait to be enhanced?", "Is it doable at no expense of other valuable traits?"...
In the list of "definite bummers", we could also include :
I don't know of any other animal that will lose all his teeth unless he cleans them three times a day with a chemical compound. And ends up losing some of them anyway.
This is a complete nonsense in the jungle. Snoring while sleeping is like waving a red-flag reading "Eat me!". So far, snoring is top of my list in "proofs of disgenic evolution of the human race".
Listing traits to be removed is incredibly boring, but what exactly could be enhanced on a human body without facing ethical issues? After all, of all the animals that I know of, human being is one of the less genetically capable one.
Let our imagination run on the subject for a while.
Body and mind improvement are for sure interesting, but maybe not possible. It could be thought that our muscles and brains can be optimised for better performance through genetic engineering but I doubt much improvement can be done. Evolution has been working at it for already 4 million years. Plus: the genetic implications are so intricate that this kind of modification is nowhere near in the future.
Night vision, on another hand, could probably easily be fitted to human eyes. Tapetum lucidum, that equip already some nocturnal animals, making their eyes shine when a source of light is directed into them, could be implemented in human eyes without major difficulties. Some primates (that is the sub-genre of human specie) have it. It has no drawbacks so far. I don't suppose that anyone is happy with the inability to see clearly in the dark.
Thermal regulation could be improved to better adapt the variation of temperature. The human body increases a little bit the metabolism when the temperature drops, but still requires the addition of garments to withstand anything colder than, say 15°C. That is the average temperature of the globe! I don't think it would be a big deal to allow the organism to burn more calories in case of cold temperature in order to achieve the metabolic temperature.
Though it might prove to be a devolution in case the human race was to return to a jungle life. This point is debatable.
Sexual drive: Evolution has fitted us with a sexual drive ensuring that the individuals of human specie strive to propagate their genes. Though this sexual drive probably proved to be very useful in a past where the humans were not many and subjected to their environment, in today's situation it is by far higher than necessary. Plus: it is entailing very antisocial behaviour. The psychological consequences of hormone manipulation is indeed very intricate, but anything can be solved, given the means and the energy. And the benefits on the social level are far greater than that of any previous man-made artifacts.
I'm still extending this list. So far I came up with:
If you have an idea, put it down in a comment, I'll add it with credits to you (I have to... for the CC licence).