It's a combination of several things. Light and composition are two of the biggest things... the subject matter can make a difference, too, although a good photographer can often take the most ordinary thing and turn it into something spectacular.
More than anything though, I think it's the photographer's ability to draw the viewer into the photograph. Something needs to grab your attention and hold you there to make it a truly good photo. This can be done through composition, subject matter, style, color, anything and everything, really.
