I have a 20D and did not upgrade to the 30D because it just looks like a 20D Mk2 to me.
There are several things that would make me upgrade to a 40D.
Sensor dust control, assuming the system on the XTi actually works.
A Digic 3 processor that will give better image quality and less noise at high ISO settings.
A true spot meter that goes narrower than the not so small spot in the 30D.
A larger buffer and/or faster processor that you allow non-stop continuous shooting in RAW mode.
A 10 or 12 MP sensor would be nice, but not at the expense of image quality.
A diagonal grid of large and small sensors that specialize in dim and bright light respectively to give greater dynamic range. (Was it Fuji or Minolta that has/had that arrangement?)
Weather sealing on EF-S lenses and bodies, such as on the 1 series cameras and some L lenses would be nice, but I don't expect to see it soon.
More frames per second would be good for some sports and action wildlife situations, but 5 fps is good enough most of the time.
A 45 point AF system would be nice in some situations, but I usually use my EOS 3 with just the centre sensor too.
I would really like to see an EF-S camera that surpasses the Nikon D2X in every way, but does not cost more than the current 30D.
