Someone who is working on listing and selling homes every day. Someone who has worked with hundreds of buyer transactions. Someone who has worked with hundreds of seller transactions as well (you need the perspective of what the seller and their agent may be thinking when they evaluate your offer).
Someone who knows the contracts, every word, phrase, clause, etc. Someone who can explain in detail or in summary exactly what you face as a buyer in Silicon Valley.
Someone who knows as many people as possible in the business. You'll do better when your agent is well known and respected in the industry. It might make the difference in a multiple offer or in the case where you need to do extensive negotiating.
Someone you can communicate with. Someone you feel is looking out for you and will find the right home for you.
Someone who is not too busy to help you. Someone who will not push you off with a less experienced staff member. Someone who has all the answers or will get them for you. Someone you can ask for references on a good CPA, Trust and Estate Attorney, Contractor, etc., knowing they'll be sending you to someone as good as they are and won't drop the ball.
It's almost a gut instinct when you meet a good agent you can work with. Call me. You can interview me, I'll tell you what I think, and you can compare as you would with any other agent. Read my answers on Trulia (I have answered a question or two for you already).
Mark Burns, Realtor
Coldwell Banker Premier - Top 2% Worldwide
President - PRDS, Contracts and Forms for Silicon Valley Residential Real Estate 2008, 2009
Chair - Region 9 for the California Association of Realtors 2009
President - Silicon Valley Association of Realtors 2007
Tour Captain - Cupertino, Sunnyvale, Santa Clara area for SilVAR weekly brokers meeting 1987-2009
DRE #00896552 licensed since 1985