Great suggestions. Thanks!
It's not the derailleur hanger, unless both hangers that I used (one on each of the 2 different bikes that the derailleur has been on) are coincidentally bent in a way so as to cause the exact same problem. And the bike has never gone down so there wasn't any opportunity to bend the hanger.
I don't have a campy cassette. It's Shimano 10 speed.
The derailleur pivot bolt is fine, to all appearances.
I followed basic installation and tuning directions (a la Park Tool and Zinn).
That leaves on the table the possibility of:
a) the cables, which were on both bikes along with the derailleur, so that may be the problem right there.
b) "cable routing to the opposite side of the pinch bolt" (vipiuva's suggestion). Uh, I'm not sure what this means, but I just routed the cable normally so I don't think I did this is the problem.
I'll replace the cables and see what happens. No one seems worried about the gearing on my cassette or the "derailleur capacity", eh?