I used to think that great photographs were the capturing of something interesting that either 1) others overlooked or 2) others were unlikely to ever see.
And there is some truth to that. Great photographs are sometimes of the overlooked. And great photographs are sometimes of difficult locations. But those alone do not make a photograph great--perhaps those are starting points.
What makes a photograph great is what the photographer injects into it. The photographer must bring something to the image. Capturing a scene is not nearly enough.
Great photographs are creations that start with a vivid and imaginative conception. Vision. Never does a great photograph start with the camera--it begins in the head and heart of the photographer well before the shutter is opened.
Photographs are a form of non-verbal communication. Like a great novel, a great photograph must clearly communicate a thought or emotion that moves the viewer. A great photograph speaks. Merely capturing a scene is not enough--the photographer must inject himself or herself into the image somehow.
If you have no ideas, opinions or emotions about what you are photographing, you'll never craft a great image. So consider what it is that you love or hate the most, and focus your efforts on photographing those things.