I think it comes down to function & cost analysis- What is it you want to do with the bike (road vs race cx vs commute), and how much you want to spend?
I'm going to point out one of the most obvious one- the wheelset(s). If you've got the cash for, say, Zipp 303s tubies, right there you are looking at ~500g (1.1lbs) off. Or, go crazy lighter, with something like these: viewtopic.php?f=5&t=100583
and you are now at ~820g (1.8lbs) off.
Of course, if you go racing, take the light, fenders, computer, cages off.