This jumps off of a comment in another thread about the mixes Mark Longworth does.
Some would define a good mix as something that's technically clean, has a full, balanced sound, applies proper use of mastering tools, etc. But then you have mixes like the ones on the early Who albums, that sound as if insane monkeys were allowed to tweak the knobs at random, but the mixes sound really cool and have a high level of energy.
The mix on some classics isn't all that great, and then you have the anomalies. For example, I find the mixes on the Stones' "Exile on Main Street" muddy and crowded, but I think they're great, and totally appropriate. It's one of my favorite Stones albums. Or take Phil Spector's "wall of sound." Probably not the idea of a great mix from a technical standpoint, but those were great mixes nonetheless.
I guess to me, a "good mix" is one that sounds like it was performed rather than just had the controls set on automatic pilot. The human touch, and all that.