If you're a seller, you're looking for the most money in the least time with the greatest ease (and environmental responsibility doesn't hurt, either). This means a solid marketing plan, a solid track record, someone who knows the market in your area, and great communication.
If you're a buyer, you're looking for someone who will make the process easy on you... someone who will spend the time to effectively communicate and learn what you're looking for so that you actually spend your time going to properties that might actually work for you. You also want someone who is a skilled negotiator to obtain the price and terms that are acceptable to you. Knowledge of the market comes in handy here, too.
Either way, the two key aspects you're looking for in an agent are market knowledge and communication / follow-up skills.
It is important to find A Realtor that is good on the web. Most buyers start lookin there first and have already veiwed your home before walking the door..