A good logo should be something simple. If you can't sketch it from memory with a pencil, then it's too complicated and will be less memorable and less recognizable. A good logo should reflect what it is representing, whether that's a company, or a project, or a wiki. A logo should work at all sizes. You might want it to be the favicon of a website at just 16 pixels square, or you might want to make a 10 foot high banner at a conference. If it can be rendered as an SVG image, then it will be easier to resize for all occasions.