There are several things that can make a good mix. Balance is relative. most importantly is if it makes the listener feel somthing when they listen to it.

When a mix comes in that is heavy or light in certain frequencies, it doesn't mean it's a bad mix, just means that there was something wrong in the mix room, (monitors, room, fatigue) that's why there is one last stage before it gets manufactured. An objective person listening to the mix in a good room with a good setup that can fix these little problems. Usually the problem is global.

A good mix should stand by itself when it comes into be mastered. It should not sound "pre mastered" but should sound good. The overall balance should be good and the peak levels around -4dBFS.

Too many times in order to please the client the mix engineer will add some bus compression and some overall EQ plus some effects to make the mix sound "mastered" but that is completely the wrong way to send a mix to a mastering engineer since he or she will not have as clean a mix to work with and may have try and undo what the mix engineer has already done. Some effect that sounds good on small near field monitors may sound way overdone on the speakers of a mastering engineer's setup. I had a mix come in that someone had run though a Behringer all in one unit that was suppose to be making the sound sound better but he used way to much of the effect on the mix and when it came here it was very apparent but he said it sounded "great" on his setup.

One of the biggest problems I face on a daily basis is that people are mixing on small near field monitors and they are adding overall eq based on what they are hearing and since the smaller near fields have limited bass response they over do the bass frequency eq. I also have trouble with rap, hip hop and reggae clients who want mega bass and have already added way too much bass to their mixes then come here and find that they are very muddy and very overdone. I can usually repair the mixes but it takes more time and sometimes I can only do so much.

Best to talk things over with the person who will be mastering your material BEFORE the final mixdown is done. We encourage clients to bring in their mixes BEFORE they come for a mastering session so we can listen to and comment on what we are hearing. It is easier for the client to change things at this level and not have to go back and remix the entire album.

FWIW

one that just pops. overcompress, itll be a straight line. undercompress stuff will fade around and everything will sound like you are outside the club on the loser list. front and centers where its at. and no im not telling you to brickwall your mix to hell, just even out the overdynamic tracks (like vocals) and a lil EQing on the conflicting parts (little scoop in the 300, lil bump behind it, roll off the sub, raise the 2-5k(maybe roll 7k+ a few db), make a very in-your-face guitar ((approx freqs))).

if its captured right, all you need is a little rearranging instrument placements (in stereo and spectrum) so they fit right and youre golden.

idealy i try and just adjust 2db on a track EQ wise, stay below 2:1 compression ratio (compressing less than a dB on average levels), hard/soft pan where needed, reverb on over dry tracks (esp mono like leads and possibly vocals, enough to notice solod but hardly in the mix), possibly some delay magic (delay left/right channels right can get a nice smooth pop tone on vocals, as well as add some stereo element).

thats usually all i do to my tracks. the only exception is i use a DI pedal for writing music, which is bland so i run it through a bass amp sim and over EQ it before its fed through. then when i finish the song, i run it through poor mans mastering (BROAD eq mid bump, brickwall to max -10dB RMS) and take it for a test drive in the car. sounds decent there, it should be good to go.