3" LCD at over 1million dots; High ISO with minimal noise; Improvement in overall feel while handling; Better Autofocus; new sensor has much better resolution.
The quality of my phots are drawtically improved over the 450D's!
My previous Canon was the 450D and it didn't have video; very impressed with the video quality.
Live View is clutsy and when using it, the overall quality of photos, especially focus is unacceptable. I won't use live view.
My first digital SLR was the Canon 350D and I haven't looked back. Since then I upgraded to the 450D and just recently the 550D. I have been extremely happy with each upgrade and I'm just getting used to all of the features of the 550D. All I can say is: "Two Thumbs Up"