K. Eric Drexler — American Scientist born on April 25, 1955,

Kim Eric Drexler is an American engineer best known for popularizing the potential of molecular nanotechnology, from the 1970s and 1980s. His 1991 doctoral thesis at Massachusetts Institute of Technology was revised and published as the book Nanosystems: Molecular Machinery Manufacturing and Computation, which received the Association of American Publishers award for Best Computer Science Book of 1992... (wikipedia)

The really big difference is that what you make with a molecular machine can be completely precise, down to the tiniest degree of detail that can exist in the world.
My greatest concern is that the emergence of this technology without the appropriate public attention and international controls could lead to an unstable arms race.
I've encountered a lot of people who sound like critics but very few who have substantive criticisms. There is a lot of skepticism, but it seems to be more a matter of inertia than it is of people having some real reason for thinking something else.
In thinking about nanotechnology today, what's most important is understanding where it leads, what nanotechnology will look like after we reach the assembler breakthrough.
Likewise nanotechnology will, once it gets under way, depend on the tools we have then and our ability to use them, and not on the steps that got us there.