Just like a Good Recording, a "Good Mix" should be without distortion AND have minimal noise. The songs tonal, spatial and inter-instrument level balance should be as close to the desired result as possible.

One thing though, a Loud Mix is not necessarily a Good Mix, so don't spend time trying to match the levels of your song to the levels of your favourite artists CD or downloaded songs. Your time and effort would be far better spent attempting to match their tonal, spatial and inter-instrument level balance rather than their final song level.

Mastering a Poor Mix will help it, but you are already starting from behind the pack.