This is for Manhattan. I don't know about the suburbs, and the rest of America.
1) A good agent is busy and will not waste their time. They will not drive you around (in Manhattan this means cab or limo) like you are the only card in the rolodex.
They will suggest you hit every open house you can for the next month to educate yourself about the market and neighborhoods so they don't waste their valuable deal-closing time... then come back, and get started.
2) A good agent is confident. They have the buyers and the sellers. They know the market, and when to expect change.
3) A good agent will lay it on the line -- tell it like it is. They have no time for lying, convincing, conniving, or any other games. It's all business. So, they will be saying things that are fact instead of what you want to hear. They might come off as coarse or stern. This is good. Don't expect to be coddled.
4) A good agent has connections... from City Hall to the Kiwanis Club to Donald Trumps favorite restaurant maitré D.
5) A good agent can spot a buyer by their shoes.
6) A good agent won't open their mouth to you without pressing a business card into your palm and reminding you that they are still waiting for your 32 year old son to move his new bride into "proper" accomodations.
7) A good agent will have a well-equipped black car waiting outside for you on discovery days.
8) A good agent will be well dressed and well informed. Their response time will be almost immediate since they are techno-savvy. Market statistics, building comparables/neighborhood comparables, buying/selling trends, recent contract data are available within the hour in most cases.
9) A good agent can provide information and response 16 hours a day. Yeah, we live our work, gladly.
10) A good agent is aware that you have a life and a business or important career to run. To that end, we are efficient.