I upgraded from a D60 to a 20D.
4 points in favour of the 20D, in order of priority:-
(1) Much better performance at high ISO's. 1600 ISO really is useable. On the D60 it's non-existant.
(2) Much faster response all round; faster writing speeds to the CF card but perhaps more importantly much faster start-up. When the camera is in stand-by mode, a touch of the shutter button bring the camera to life almost instantly. With the D60 there was a significant delay that potentially would result in a lost shot.
(3) Availability of a true wide-angle lens, the Canon 10-22mm. This is equivalent to the 16-35mm on the Canon full frame 35mm.
(4) Marginal increase in resolution. Not significant, but any increase is better than no increase
