The overall visuals of the groups is pleasing to the eyes.
The dancing is on point. Very synchronized, crisp and clean. They can dance and sing at the same time.
Video production on their music videos are very high quality, colourful and often have interesting concepts.
There is a lot of marketing and promotions, some of the members in the group go on variety shows and youtube to promote their latest releases. At the same time you get to see their personalities come through as well.
For me personally though, I like the fact that they mix other genres into the songs. By doing so you don't also know where the song will go or how it will turn out to be.
The production team is good at pulling off a song with very contrasting and different genres all into one.
Why do I think it's famous? Because the people who produce it create idols out of the stars, and people stick to those idols.
What do I think about it? As a kind of music I personally dislike it, but I especially hate K-Pop due to the horrendous ways in which the idols are treated and how unethical the whole scene is. It's tantamount to abuse and should be shut down or changed for the better.
I'm a cynic, and I think learning about the "mass-production factory line" work behind the scenes gave me a bad impression of that music scene, if you can call it music at that point. I know pop music can be foreboding like that in general, but it gets really bad in the asian pop scene as far as I've learned.
Plus the music itself doesn't really appeal to me one way or another, so hey