I went from the D30 to a 20D about a year ago -- it's a world of difference, in some ways, but maybe not as huge step as I thought it should be.
One of the major reasons why I upgraded was that the D30 didn't auto-fucus wery well in low-light conditions -- well, the 20D does a slightly better job but not as good as I wanted. I later taught myself to use the support light of an extended flash to help the autofocus, a technice that probably would have worked on my old camera as well...
My favourite feature is that fact that I don't have to spend a lot of tile rotating all vertical pictures in my computer afterward (the D30 didn't have an oprientation sensor).
Basically, the D20 allows me to take pictures faster (e.g. it doesn't lock up while writing the last couple of pictures to the memory card), it allows me to more easily inspect my pictures due to a much better zoom features. etc. However, the D30 and D60 are still great cameras, they still take great pictures, if you already have one (or can get one at a bargin price) it's a good way to start learning what SLR photograpthy is all about.
