It's important to understand that molecular manufacturing implies exponential manufacturing—the ability to rapidly build as many desktop nanofactories as you have the resources for. Starting with one nanofactory, someone could build thousands of additional nanofactories in a day or less, at very low cost. This means that projects of almost any size can be accomplished quickly.
Those who have access to the technology could use it to build a surveillance system to track six billion people, weapons systems far more powerful than the world's combined conventional forces, construction on a planetary scale, or spaceflight as easy as airplane flight is today.
Massive projects aren't always bad. Rapid construction could allow us to build environmental remediation technologies on a huge scale. Researchers at Los Alamos National Laboratory are suggesting that equipment could be built to remove significant quantities of carbon dioxide directly from the atmosphere. With molecular manufacturing, this could be done far more quickly, easily, and inexpensively.
In addition to being powerful, the technology also will be deft and exquisite. Medical research and treatment will advance rapidly, given access to nearly unlimited numbers of medical robots and sensors that are smaller than a cell.
This only scratches the surface. Molecular manufacturing has as many implications as electricity, computers, and gasoline engines—combined.