I love designing logos - a good logo works in black/white as well as colour and should clearly convey the business name. A logo needs to be legible in any size from tiny to large. Also bare in mind that part of a logo might need to be turned into an icon or favicon which can be used on websites and social media channels.
To me, a legendary logo makes creative use of white/negative space. While this might not always be visible at first sight but will reveal itself in due course to the curious observer. One of my all time favorites is the arrow in the FedEx logo.