I think it depends on the level of investment being put into the activity - here, listening to music.

I had one friend who was very, very into music. He read music blogs, he knew all the names of all the members of his favorite bands (and there were many), he listened to TONS of current albums, and he went to a lot of shows as well. When he listened to music, I could tell from our conversations that he listened intently and retained a great deal more about a single song from a single listen than I often did after listening to a song several times. I think that was partially because, as a person interested in music, he was listening for and able to hear a lot of finer details that the lay-listener didn't know or care to listen for/hear. I think it was also because he was actively listening, not just passively turning on a track and letting it play while he did other things in the background.

It's like - I hear people talk about "production." "This track was minimally produced and has a very raw sound," or "This track is very produced," or more specifically, talking about styles of production - like Dr. Dre and what makes him a great producer and why.

I can kind of hear production. I can tell when a track is more produced over less. But I am very hard pressed to talk about details of production, or production style, or what it is that makes Dre so great at being a music producer or how he does it. I think people who really care about music and are really good at it, though, listen on a different level and are able to differentiate between things like quality of underlying vocals, quality of the melody/bass line/drum line/etc, and where "production" starts and what it does in a given song.

So I would think someone who writes music reviews would listen to music more on that elevated kind of level, and can see why it could be considered a hobby, because it would be a lot more active than what the average person does.

Then again I see your point, where saying "Listening to music" is a hobby is similar to saying "I like to have fun" when someone asks what you do in your spare time. Well, yes. Everyone likes to have fun. Liking to have fun says little to nothing about a person. It is how they find their fun that is more interesting and important.

Do explain if you can.

My pleasure! Let's go by parts:

most of all people on the planet listen to music in some capacity

That's right. If their ears work, people listen to music.

whether it's great or small doesn't matter. It's like putting "watching TV" or "shopping for groceries

I get you. You're talking about the casual listeners. These are the people who listen to whatever music is playing on the radio at the moment or whatever is popular right now. When I'm talking to someone and they say "oh, I like all kinds of music" and they don't go in detail (genres, bands, etc), I identify that person as a casual listener. No problem with that, this is how most people start.

After that, you get into a single genre. I live in a place where those who care about music are crazy about rock and heavy metal. They listen to the most famous bands of their preferred subgenres. You could say this is the second level of the casual listener, along with the die-hard fan of a single band.

All are proper activities in which not the absolute majority of the developed world is involved. Hobbies are activities that distinguish you, that make you you, and I can't grasp how doing that thing that 90% of the developed world population does makes you any different.

Hmm... I think you should take a look at this list I shared with yellowoftops.

This is the difference between casual listeners and people who love listening to music. I started that list around 2 years ago. I usually go through the entire artist's discography, listening to the albums front to back. It used to be a single genre for the first 100-150 albums, but then I felt an urge to diversify a bit. Now, I listen to around 2-3 albums a day: if you look at the end of the list, you'll see that right now I'm oscillating between metal and disco.

About doing something that 90% of the developed world population does... well, we get a lot of great music with each passing day, and one simply cannot listen to ALL the music out there. I tried finding some bands in common with the people I used to talk about music, but it wasn't of any use: all the bands I knew were unknown to them, and they didn't know any of the bands I liked. After the 300 albums mark, I just stopped name-dropping the bands I liked, it's just so many! As time went by, I noticed my taste got broader and more specialized.

In closing, it classifies as a hobby for me because I get a craving for new music whenever I stop looking for new stuff. And there are the "oh, there's nothing interesting out there" cycles every 3 months.