Live Rock, Macro algea in the sump. I have used 100 micron bags to filter out large particles when the tank was new, now that is even gone. And also if there is a problem carbon.
Agree with the above. Also if you don't have a skimmer already it is a great place to hide all of the equipment that is kept HOB of a non-sump tank like heaters etc. Some people even make their sumps display sumps. I use mine for the former not the latter.