I'm gonna guess "shoes".

Seriously, though, I can go a week with just a carry-on also. One fifty pound bag could get me through two weeks easily. I don't wear shirts twice but I'll wear slacks or pants twice during the trip. One or two pairs of shoes max (and I'm wearing one pair).

The main reason I think many people have so much luggage is that they try to bring "something for every occasion." Well, if you know where you are going and what you are doing in advance then you should be able to choose appropriately in advance for your needs.