Look at sites similar to the one you plan to build. What about their navigation, tools, etc., make the site easier to use? What was lacking or confusing that could be added or changed to make the user experience better?
Listings... listings... listings... and the ability to easily search them.
Unfortunately, the Board of Realtors in most towns control the MLSs and do not want to share that info. If they do then they setup all kinds of rules limiting what can/cannot be shown. Interfacing with them is often difficult since these systems in many cases are old, antiquated, home grown systems... and frequently don't even have real developers supporting them.
Mapping capabilities to display search results on a map of the area.
Home valuation capabilities to see the value of the consumer's current home (if applicable) as well as homes in the search results or neighboring homes.