Most people use ISO 100 for outdoor photography, especially on bright
sunny days. ISO 200 for indoor, 400 and 800 for low light situations,
and 1600 for very low light where the shot would be impossible to
get any other way. If you find you don't have enough light to get a
shot, you basically have two options - higher ISO or a tripod.
People who are "prejudiced" in favor of the D30 (and I include myself
in that category) particularly like the ISO 100 performance, and the
fact that the full gamut of 100-1600 is available. This makes the
transition from film feel more natural. I can't tell you how many
times I've been at some event, gone from taking pictures outdoors to
indoors and back again, and was grateful that I could switch back and
forth between 100 and 200, or 100 and 400, or whatever. It's very
addictive!
Another interesting point is that ISO performance has become sort of a
yardstick that gets applied to each new D-SLR as it comes out. The
D30 was really the first one to offer a 100-1600 range that "just
worked" the way you would expect it to. The upcoming D60 does not
have 1600, for reasons that remain mysterious. The 1D has 100-3200,
but the 100 and 3200 "endpoints" are implemented via a special "mode"
that sacrifices a small (but noticeable) amount of image quality.
Anyways, thanks for asking such a good, basic question. I think this
forum needs more of that. Thanks.