It was quite gratifying to discover that I already knew all that! I actually learned it all through experience and simply observing what makes a good photograph. I wasn't even aware that the "not centering the horizon" thing was a formal rule, but it's something I found myself avoiding without even realising it (especially when going wide).

Excellent advice all round though - especially the stuff about light. Light really is the essence of a photograph. I don't even bother turning the camera on if the light's wrong. As soon as there's promise of decent illumination, I'm out of the house like a shot every time. I think this is the most often overlooked aspect of photography, with many people mistakenly assuming that if you point a camera at something pretty it will automatically make a good compostion.