My answer in regards to the most important factor in house pricing would be: Location.
If the house is located in a bad neighborhood or is simply a large house surrounded by small houses, the value of the house goes does (meaning the price decreases). Also, the average price of the surrounding houses affects the house's price. Example: if you are going to sell your house which is small in a nice neighborhood with large houses, the value (price) will go up on your house.
So in short, location is often a large determination in house pricing.
Hope this helps! :)