The sensor for one, the 13.6mp over 10 and RAW1 and this from camera lab:Canon EOS 50D verdict

The Canon EOS 50D is a feature-packed semi-pro DSLR which takes the already capable 40D, increases the pixel count by 50%, quadruples the sensitivity and packs in all the latest features – bar a movie mode. As such it’s a very impressive specification, and one which on the whole lives up to its promise.

Kicking-off with resolution, the EOS 50D packs more pixels into its cropped sensor than any model to date, so there’s obviously concerns over noise levels. But Canon’s bold claims of matching the noise levels of the earlier EOS 40D were confirmed in our High ISO tests. From 100 to 1600 ISO, the EOS 50D’s output really is very similar to that from the EOS 40D when viewed at 100% on a pixel level. At 3200 ISO, the 50D applies greater noise reduction, but tone it down a notch and again it looks similar to the 40D. Admittedly the 50D’s 6400 and 12800 ISO modes are a step too far, but it’s an impressive achievement to boost the total pixels by 50% without any perceptible compromise in noise levels over its 10 Megapixel predecessor at the same sensitivities.

It’s also important to remember these comparisons were made at 100%, viewing each pixel at 1:1 on a computer monitor. If you print images from the 40D and 50D at the same size, the 50D’s higher pixel count means any artefacts would appear smaller, so in this respect it has the advantage.