I started with a 350D and upgraded to a 30D, then to a 40D.
My reasons were:
350D - 30D
mainly financial, got a good deal on a 30D / 24-105 L combo and got a good deal selling my 350D & grip. However, liked the balance, spot metering and build quality. Really liked the 1/3 ISO stops - I do a lot of low light theatre photography, so this came in useful. I really got this for the 24-105IS, the deal was good enough that the 30D was effectively £ 100 after I'd sold the 350D and assumed the typical s/h market price of the 24-105L
30D - 40D
again financial reasons came into it.
superior AF capabilities (for motorsport)
superior low light focussing (for theatre)