What affects a paint job?

Many things, such as: * Weather (rain, sleet snow) * Surface you are painting (is the wood chipped?, is the wall damaged) * How you mix the paint (did i add too much of this? or not enough) * The type of brush you use (is this thing to small for this big of a job?) * Surroundings (will this get in the way? will paint drip on this?) * Sunlight (will the sun make this side dry faster than that side?)