That other thread made me wonder about the below question.  This includes second-guessing some songs/albums that I think are well produced just because they weren't mentioned (yet?) like No Doubt's Return Of Saturn.
 
Since musical preferences can be so subjective, what do YOU consider to be a well produced song?  Great separation of each instrument & vocal?  Fewer effects?  More headroom?  Something else?

I think it really starts on the front end.  Talented musicians and great songs.  Then, clean well recorded tracks.  

Short answer...
 
You can hear the music... not the "mix".
 
Long answer may or may not be forthcoming.

Each step has it's opportunity to enhance or screw up a tune.  So my answer is the best produced songs are the ones that none of the steps were either overly or poorly done.
 

I like when each instrument/voice has its own space and can be heard individually if you listen closely. The bass may not have the spotlight most of the time(sorry Bapu), but I still want to be able to hear what it's doing. I like a song that moves dynamically with the emotion of the song as well.

Edited for spelling. (:

The music is well defined. Meaning I can hear everything and it sounds like it is all in its proper place, then again that may be more of a good arrangement than a good mix, or possibly both. Tuesday I was driving my daughter to the airport and I was flipping around on Sirius/XM, thinking about the question of "songs that you think are a good mix". Steely Dan's song, My Old School came on. When the instrumental part with the guitar solo and horn section was playing I thought, "wow, this is a great mix" then I also thought that it is probably more of a great arrangement. It is shockingly good, IMO. It is right there with the best I've heard.
So, I don't know if it is more mix or arrangement or a combination that makes it stand out to me. Obviously, when there are vocals, the instruments should compliment the vocalist, guitar shouldn't be interfering.
Mainly, things should not be stepping on each other. Everything in the pocket. Not too much compression.
I have a set of EQs I can scroll through. I generally like things pretty flat but sometimes I like to bump the bass just a little and the highs just a little (Car stereo). I generally don't like to use Loudness and I think if I have to use Loudness to make a song sound good, it isn't a good mix. If Flat with some volume sounds good, it is a good mix. I used to always bump the highs, quite a lot. That is probably why I have such tinnitus, but highs sound good to me, more natural. Bass sounds unnatural to me. It may be different for you.
These days I have to use the EQ on my right monitor to bump the high a little because I can't hear highs as well with my right ear. I've almost always kept my guitar amps to my right and you know, loud is GOOD, right? What?
 
Anyway, I think the best mixes are songs done by excellent musicians playing exceptional arrangements. That is what I think is most responsible for a well produced song.
 
J

A song that when I get to the end I think, "that's great I'll have another listen". Could mean well mixed, well written, well played or well mastered. Just depends. Any, either, or all of these criteria make for a song that's worth going back for another listen.

if is sounds good it is good.

Another factor - the production matches the content/concept. 
 
An example of that in my opinion would be Marilyn Manson's Mechanical Animal. See, that's not necessarily a type of sound I'd be attracted to otherwise, or that would work for something else. But there's such a symbiosis between the songs and the production values. KISS' Creatures of the Night is another one.
 
It may seems obvious, but lately, I seem to be hearing a lot of technically very-well produced music which all sound the same and has absolutely no identity. It's very formulaic. It's as if all those people watched the same tutorials and read the same books, instead of forging a sonic personality through a long apprenticeship in a conventional studio. 
 
Plus everyone's using the same plug-ins, the same sample libraries and all. A lot of what made each record unique has been systematically removed from the equation.
 
Anyway - I tend to prefer things that stand out a bit. Unexpected or even odd.
 
That plus all that's been mentioned before.