It’s not that easy to explain, but the main thing is being able to take your audience on a “Journey,” and being able to “Crowd Read.”
Taking your audience on a Journey means that you play songs that mean something, you should be able to tell a musical story through your music. A mix should contain ups and downs, happy and sad, slow and fast, and so on.
And, Crowd Reading has to do with looking at a crowd, and figuring out what songs to play. It’s a skill that takes a lot of time to obtain.
Lastly, yes, gear and transitions are kinda part of it, but it’s mostly the stuff I said.