My Buddy used to tell me never to shoot under 1/125 shutter speed, or you chance movement blur. I think the new digitals are more forgiving in that manner than the film cameras.
Yes lenses, holy cow, you can spend a small fortune on glass, but, ya gotta have it with an SLR, digital or otherwise.
Do you shoot in RAW? I hear that you never have to worry about degradation when processing a RAW image.