This may have been covered, but here's my $0.02 on getting high synthy numbers:
Photosynth works off of parallax. In an ideal world every feature of the scene will have been photographed from at least three vantage points. This gives three separate sets of angles to establish the location of that point in 3D space and establish the location of the camera for each image.
If you want to do a panoramic synth, do three panoramas of the same scene from three locations. Make sure every part of each panorama overlaps with the other panoramas. The wider the angle between your three locations and the subject, the tighter the fit to each point's location in space, and the better the solution for the camera location.
As has already been pointed out, you need texture so that key points can be established between images. Featureless expanses of color, objects that move, WATER, these things do not contribute at all to a synth.