In general, materials which require extreme production conditions, or devices require great levels of detail to function, could never be produced by a 3D printer. These include:
- Computer chips (the transistors are far to small to be printed; it takes hundreds of steps to make a computer chip and the only reason they don't cost millions per chip is because we produce them in batches of thousands).
- Components made of metal alloys with very high melting points (the printer wouldn't be able to melt them to print them)
- Crystals such as diamond, which require enormous amounts of pressure to form.
In addition, I believe that it will always be cheaper and easier to mass-produce some things with specialized machines than with a 3D printer. If you have a very specific job that needs doing, a tool designed specifically for that job will almost always work better than a generic tool made to handle a wide range of jobs. That said, 3D printers can greatly accelerate the design process, and there will be many instances where it's easier or cheaper just to 3D print a medium-quality product, as opposed to purchasing the highest-quality version.