If technology proceeds at its current pace for a few more decades, we
will have the ability to control the position and bonding of
individual atoms--with a lot more flexibility than current "wet"
chemistry allows. This has significant implications. I know of no
well-informed counter-arguments to anything I'm writing here.
Here is a fictional interview I wrote recently that discusses the technology and some social issues in a more informal style.
Cheap Fast Manufacturing
If we can stick atoms together to make rigid structures such as diamond, we should be able to make atomically precise factories that can make copies of themselves. Bacteria can reproduce (double) in 15 minutes. Potatoes cost less than a dollar a pound. Now imagine a home appliance that can build a wide range of products--including a copy of itself--producing its own weight in output every hour, building with a range of materials including diamond, at a cost of a dollar a pound.
Improved Materials
If you scale up this tabletop factory, you can produce construction equipment for the same low cost--better built and stronger than today's products, because diamond is 10 times stronger than steel. The manufacturing process should be a lot cleaner than today's processes. And if you need to build more stuff, you just take your factory off-line for one hour, have it build a copy of itself, and double your production capacity.
Medicine
If you tell the factories to build smaller stuff, it should be possible to build robots that can fit *inside* cells and repair (or change) DNA under direct control. You've probably read articles about micromachines that will be able to scrape plaque off the walls of arteries; nanomachines will have far more subtle and powerful abilities.
Power And Solar Energy
There are solid engineering predictions that nanomachines should be able to convert between mechanical, chemical, and electrical energy with almost 100% efficiency and extremely high power density. Power storage will become far easier and more reliable, allowing the use of solar energy for virtually all energy needs. With more energy available, and basically unlimited manufacturing capacity, problems such as fresh water are not worrysome.
Another Computer Revolution
Computers will be more powerful by a factor of about 1,000,000,000. We can't predict what they will be used for; who could have predicted desktop computers and the World Wide Web back in 1950? But human-class artificial intelligence becomes a lot easier when your desktop computer has the raw power to simulate every neuron in a human brain.
Easy Space Travel
Aerospace advances due to improved materials and manufacturing will allow spaceships to be about as common as cars are today. We could literally lift the entire population of the earth into space, with minimal environmental impact and low financial cost.
Easy on the Environment
What are the environmental implications? Well, nanotechnology will allow us to build almost anything we can imagine today out of almost pure carbon, with no industrial waste. If you don't want something anymore, just burn it--someone else will pull the CO2 out of the air and make something useful with it. In fact, there may be enough products made that the atmospheric CO2 content will be depleted! In the best possibility, we will be able to stop polluting entirely, by using clean industry and recycling everything. If the Earth starts to get too crowded, we can leave--but if the correlation between technology and small family size continues, the population may stabilize once everyone has access to nanotechnology.
...Or Maybe Not
On the other hand, with the ability to create double the manufacturing capacity every hour (that's a factor of sixteen million in a day, or two hundred fifty trillion in two days), we could literally strip-mine the whole Earth in a week, though I don't know why we'd want to. Advances in biotechnology would make cloning trivial, and perhaps even allow the creation of adult organisms (including people) in a matter of days. So we could have a massive population explosion.
New Problems--But Not Today's
Thinking about the effects of nanotechnology requires a different mindset in some ways. For example, a common response to the idea of indefinite lifespan is, "Wouldn't that cause a population problem?" The fact is that it really doesn't matter whether people die after 60 years, or 600, or never. What is far more important is how many new humans they create. With nanotech, people will have the choice to create zero or 1,000 offspring. Today, resources are limited, and the allocation of resources is a zero-sum game. With nanotech, there will still be some things that are limited: land; whatever is seen as a luxury; whatever is limited by those in power. But there is no need for water or food to be limited, because a few square meters of solar energy collection will allow the creation of pure, fresh food and water from recycled waste. There is no need for posessions to be limited; 10 pounds of well-programmed nanomatter is incredible wealth by today's standards. Most people around the world own 10 pounds of carbon. However, the disparity between the "rich" people and the "poor" people will probably increase, though the "poor" may well be richer than today's "rich". This may or may not cause massive social problems. Finally, nanotech will allow the creation of weapons more threatening than anything we have today; there's no way to tell whether defensive technology will keep pace. On the other hand, atomic technology and biological weapons will be a lot less scary--their effects will be much easier to deal with. Genetic damage will be no concern at all--a GATTACA scenario of designed, error-checked genomes is quite plausible.
14000 BC to 2000 AD = 2000 AD to 2050 AD
Very cheap food production; improved materials; control of disease; vastly increased manufacturing capacity; better access to resources and control of energy; improved aerospace transportation; computer technology as far from today's as ours is from 1950; the ability to program genetics like we do computers. In short, nanotechnology has the potential to make as much difference as the discovery of agriculture, steel, germ theory, the assembly line, the colonization of America, electricity, the airplane, computers, and genetic engineering, all put together--and to do it without further environmental damage! And all of this is expected to happen within the next few decades.
For more information, check out http://www.foresight.org.