I am a week or so from purchasing a brand new Giant XtC-1. The stock bike seems sweet enough for me, but I'm tempted to make a few customizations: switching the Hayes Stroker Ryde disc brakes for Cane Creek Direct Curve 3 v-brakes; replacing the 9-speed cassette with a 6-, 7- or 8-speed cassette (Who makes quality 6- and 7-speed cassettes?); and adding Shimano XTM760 hubs in place of the Formula 32H disc hubs.
I've been riding an older Giant with v-brakes and a 6-speed cassette for the past eight years and manage to get along fast enough, so I figure upgrading those parts on a new bike would work out well. It would, by my math, take the total weight of the bike from 29.3 pounds to about 27 pounds.
Switching from disc to V brakes would be a big weight saving, but will only be worth it if you dont do mud. Changing to 6 or 7 speed rear end will be complicated (hubs and shifters) with hardly any benefits.