Just out of curiosity...

[Alex_B]

MP are not really the main feature.

But image quality varies alot from camera to camera. take 8 MP images from several cameras, and they will be very different with respect to image quality, even if the same lenses have been used. Talking of dynamical range and other factors ...


The rest of the difference is mainly features, if you need some of them, you have to pay for them, if you can live without them, good for you. It really depends. (Talking of fps, weather sealing, mechanical sturdiness, proper spot metering, ... ).

 

[Big Mike]

A good experiment would be to go into a camera store and compare something like a Rebel XTi to a 1D mk III. You will soon realize the rather large difference in construction and build quality. Pro bodies are built like tanks because that's what pro photographers require. They can't always afford to baby their cameras and they can't afford to have a camera break on them while on the job.
The mid level cameras are somewhere in between.

Compare it to cars. Realistically, everyone could probably get by with a Toyota Tercel...but plenty of people buy big trucks or SUVs. Plenty of people drive luxury vehicles that are very expensive.
It's no different with cameras. Sometimes you need the more expensive models...and sometimes you buy it, just because you want it and you can afford it.

 

[Jeff Canes]

Personally prefer the rear control dial on the 10D and up over the toggle buttons used on the Rebel models. Plus build quality.

 

[Jus7 A Phas3]

If this is your first DSLR i would not get a 1 or 2 grand camrea. DSLR's become obsolete with in a year or two. Get a decent body, i dont know anything about cannon but for nikon maybe i d40x or a D80 and get some good lenses that will last you years. Then when you get really experinced in photography get that 1k body.