What do you think makes a song well produced?

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

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.
 

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

if is sounds good it is good.