Wow many great answers. For both Buyers and Sellers Definitely an appealing and user friendly website. Then you need a place for them to see your sphere interact with you so a complete Truila, Zillow and Facebook. Not just a facebook page but a website on facebook with a wall that informs and uses video, podcasts (playable at the facebook level) pictures, webinars and more.
People want to see prominence and expertise. Specifically they want you to bring up solutions and good points of view. SHOW them why they should use them not just show them an award for being the top salesperson the 2nd Tuesday of the week.
First page placement for you, your number, your website and questions you answer show clients that you are ahead of the pack. So many facebook "pages" are just plain with posts of other sites' posts with no real insight as to why they should use you.
For sellers the same things apply but they want to see that your website or social media has an audience for their area. We are in the process to moving our site to someone that guarantees first page placement so soon we will be able to farm our markets easier because sellers look at how well their listings will get in front of people on search engines for their area.
There is so much more but this should have you thinking. The web turned a corner this year...its what you have to say and having that seen everywhere that attracts buyers and seller.