That sweet spot could be described as the place where inexpensive 3D printers could be obtained by any small business or any interested consumer, where supplies and designs are easily found and used. The 2D printing market of today gives us hints of what might be: inexpensive devices in homes for basic and immediate prints, more capable devices shared by users of service bureaus and monster printers hidden in the back rooms of major companies.
We're obviously not there yet. But we're heading that way, and from two directions.
First, from the top we have the commercial manufacturers, such as ZCorp, Dimension, Objet, Solido, 3D Systems, EnvisionTEC and others gradually building 3D printers that are higher in capability and lower in cost. At some point one of them will hit that sweet spot.
Meanwhile, from the bottom we have hobbyist / hacker initiatives such as MakerBot, RepRap, Fab@Home, The Rap-Man and others gradually producing 3D printers that are higher in capability and very low in cost. At some point one of them will hit that sweet spot.
See what we mean? It's a race. Who do you think will win?