Nostalgia and the memory our perception of Wayne lead many of us who are a bit aged to view the Duke as a great actor.
Truth be told, John Wayne really wasn't a great actor. Wooden to say the least. It's often been said he played only one character: John Wayne.
Absolutely he was in some good movies. He just was not a strong actor. He lacked depth and versatility.
He was an actor from a different time. He was good because the film companies said he was good. And that worked back then.
Will have to agree in part and disagree in part. He developed the Wayne character over his lifetime in film, true. No one can touch him there, of course.
In terms of his skill as an actor, I think he is underrated. He was at his best when a really strong director was at the helm. Consider the scene in "She Wore a Yellow Ribbon", a John Ford film, when he is turning over the troop on his retirement. Never mind that he's playing a character much older than his age at the time, he brings quite a lot of depth into that scene. His performance is subtle and he conveys so much. It's a poignant and memorable moment.
We think of John Wayne only being able to play John Wayne because that is the role he chose to play most often...he knew he was a brand, knew what his public wanted from him, and offered it with several movies a year, year after year. And, year after year, he became more seasoned at his craft. He was extraordinarily hard working.
Today, many movie stars like to explore different avenues, do different things. They avoid, consciously, developing their own brand.
Wayne accepted his brand and would avoid projects or choices that ran against it. For instance, when filming the gunfight in "The Shootist" he refused to follow the action exactly in the book, where his character mercilessly shoots one of the bad guys in the back. Why? Even though it might have been a strong choice, artistically, and revealing of character, he would not do something that ran against the John Wayne brand.
It is because he stayed so true to his brand throughout his career that he is so beloved.
It is also why we will never know all that he might have been capable of doing, as an actor, if he had not remained so true to that brand, and so loyal to the expectations of his public.