The "shift" button for aperture. That was my biggest issue with the XT. I just couldn't get my coordination down to hit it instead of the self-timer button. I couldn't shoot M fast enough, so stuck with Av mode most of the time. M came natural once I upgraded to the 30D.
Ok, but that's an ergonomic issue and doesn't really limit progress. It's not as that you can't do it, it's just different and more convenient in the two digit cameras.
I'm not trying to be an a$$ here and I love my 5D and my 400D and soon I hope my 40D. They each have strengths and weaknesses but I have found very little difference in capabilities between them. The 3 digit cameras are not training cameras nor will they limit your abilities. They are full featured and just as capable in nearly all respects as the 2 digit cameras. They will deliver identical image quality. To me it's all in the feel of the camera.


