Eugenics is a very simple idea, but the implementation of eugenic programs are so difficult and complex that even if you could achieve a successful eugenics program, we wouldn't want to. In fact our current society is so against the principals of eugenics, that we are actually actively disgenic.
Before we get to far, lets look at some definitions so we are all talking about the same thing:
Eugenics- literally this mean "pertaining to good genes" Eu-"good", gen- "genes", -ic -"pertaining to".
Eugenics movements / Programs - Programs designed to take natural selection into our own hands and modify the gene pool of a populaton with intent to make it better by selective breeding, sterilization, or extermination based on genetic characteristics.
Disgenic - Describes something that works in such a manner as to be bad for a gene pool either by eliminating good traits, or promoting bad ones.
Now that we are all talking about the same thing, we have to ask ourselves what is good for us genetically? This is the major problem with genetics. Who decides what is good an bad, and how are we sure. Even aside from the obvious flaws that we see in situations like Hitler trying to produce the perfect race by killing everyone who didn't fit the bill, there are genetic issues to consider as well.
In a broad sense, by definition eugenics narrows the gene pool which weakens it. Having a broad gene pool (lots of different genes) not only allows that population to beter adapt to changes, but can also mask genetic problems. The best example of this is the world dog population. Any vet will tell you, if you want a healthy dog that will live a long life, your best bet is to buy a mut. The breeding programs required to produce the various breeds of dogs carried a lot of bad stuff along with the traits they were selecting for. For example, German Shepherds are prone to hip disorders, Dalmatians are prone to epilepsy among other things, Great Danes are so large that they have heart problems and live especially short lives, and the list goes on.
Effects of genetic bottlenecks can be seen in the human race as well when we look at royal lineages which inbreed to keep the money in the family as it were. In addition to keeping the power together they were also able to keep things like hemophilia concentrated in their population.
In addition to bad traits following good, we must understand that sometimes what we think is bad may be an advantage later on. For example Sickle Cell Anemia is fatal if you get it from both parents, and it is no picnic if you only get it from one side. On the other had, it does offer immunity to malaria which is one of the reasons it is so prevalent in areas where malaria is endemic.
Being tall is often advantageous in business and sports, but exceptionally tall people tend to have serious back problems as they get older, and people with gigantism tend to live shorter lives in much the same way the Great Danes that we discussed earlier do.
Because of all this we have widely dismissed the idea of practicing eugenics. The question now is what to we do about the fact that our modern medicine is disgenic. Until recent history, our gene pool was cleansed by the fact that the most serious genetic disorders killed you before you could breed, and were therefor kept at low levels in the population. With modern medicine, we are able to treat many of these disorders, and provide longer lives, which while it doesn't promote these "bad genes" it allows them to continue on in our gene pool in higher numbers.
We live in a society where we can't stand to see human suffering, and everyone is provided for, and everyone is cared for at least reasonably well by our health care system and social programs. While I wouldn't change that for the world, we must at least acknowledge that natural selection, except in the extreme cases, is no longer working in our society. Future advances will most likely not come from our genetics, but from our technology.