Without a doubt — PRICE.
Price is Key. No matter how much advertising is done by a home seller and their agent, a home that's priced too much over current market value is just not going to sell. If it's priced too high you may get viewings by buyers and their agents, but you're attracting buyers that are looking in a particular price range and your home won't compare favourably with the current (and recently sold) competition in that price range. Buyers and their REALTORS® are looking at a number of homes and it becomes very obvious very quickly which homes are overpriced.
If the home is priced right, it will receive consideration from all prospective buyers and their REALTORS®. A good REALTOR® will help their prospective buyer understand the value in your home and likewise try to diswade a client from overpaying for an overpriced home (they actually have a legal obligation to do this).
A home that is underpriced will possibly end up in competing offers and...