A good mix will be interesting but not distracting. You should be able to turn it up loud without any frequencies that hurt your ears, and you should be able to hear the details of the mix even if you haven't turned it up loud.
In order for this to happen, each instrument needs its own space. During recording, I'm often thinking about where best in the mix to put whatever I'm recording. For example... If I want a more "distant" sound on an acoustic guitar, I might actually put the microphone further away from that acoustic guitar, to help give that effect.
Then, while mixing the sounds together, I can experiment with effects and dynamic processors to make the right sounds pop when I want them to. If a mix is done well, it should having you into the song and not detracted by the mix.
I go for goosebumps! That's how I know the balance is feeling right, because I literally FEEL it. Invoking an emotion is important in my book, and a good mix will lend itself to invoking the emotion of the song.
Is the song mysterious?... Is it happy? Is it sad?... Or, is it supposed to feel big and powerful? Is it intense, or is it just energetic?
These are all questions I ask myself during the pre-production and recording process in order to aid the mixing process. There's a lot of creativity required to get a mix to sound exciting, well-balanced, and professional. Ultimately, the listener should be consumed by the song, and inevitably, that requires a good mix!