I am a mortgage broker so I consider my opinion from a home buyer or seller's perspective since I do not ever deal with real estate directly. I already have a realtor that I work with consistently but the things that I look for from her and the reason I chose her:
1) Compatibility - our personalities fit, she listened to my goals and needs, and found properties accordingly.
2) Communication - this is key, it irritates me to no end when things occur and I am not updated or if I call and I do not get a return call. Updates are critical in a high-pace, high-stress experience. And considering how much money is at stake, communication is even more important than in any other case.
3) Network - the amount of people in the realtor's network, if she is well-liked in the real estate field, it all matters. I would never want to work with a realtor where his or her reputation hurt the chances of my property getting sold.
4) Creativity - there are times, especially nowadays, when thinking of new ways to sell a house, reach the most potential buyers, etc. is extremely important.
5) Skills in Negotiating - as part of the homebuying or home selling process, I know that the realtor will be presented with many times when the option of bargaining for more concessions or strategizing for a higher offer price, will be important.
6) Dedication - after speaking to a few referrals you can get a pretty good idea of how hard the realtor will work for you--whether they will post my home's listing on Craigslist, MLS consistently, advertise it well, target it well.
I mean there is so much more to choosing a realtor, but trust, compatibility, and communication I would say is the most important. You can get a feel for the realtor's experience and knowledge of real estate and the area through conversations and my own research and everything else tends to fall into place.