That you can succeed at a task by describing what you are doing or how you are approaching a problem without needing a die roll.
Characters are an avatar for you to explore a fantasy world more so than a full persona.
EDIT: Just realized I was replying to a necro thread. Ooops.
Agree with your whole list, I just want to expand on these for people who hadn't played them.
In current D&D, we play characters that can do fantastic things, many of which the players can't do. And the flip side is that there is an acknowledgement there are some things the players can do that the characters can't. For instance my 8 Charisma barbarian without skills will not be an effective face of the party.
But skills didn't exist in the early game. They came in as optional rules, and not even in the core books.
There's an old saying "No one ever fell off a horse before there was a Ride skill". It means that your characters were assumed competent at things that made sense, and without a mechanic like skills, there was a lot less mechanical checks.
There was no Investigate skill to find a hidden compartment in a desk. You described pulling the draws out, checking their height and length to see if there's unexplained room. You might knock on solid parts to listen if they were really hollow.
Even where there were checks, descriptions could trump it. Just you might have a 1 in 6 chance to find a secret door (more if you were an elf), but if you manipulated the torch sconce that controlled it, the door would open regardless of that check.
So there was a lot of hands on, a lot of player skills fitting in for character skills (since there were no character skills) fuzzing that line between player and character.
Think of the simplest way to do Riddles in 5e - just rely on player knowledge and ingenuity to figure them out. Now apply that to 85% of things that would be skill checks in modern play, because there was no mechanics to say how good the characters were at them.