I reckon to be considered a "great" angler, you've got to have a few things:
1. A deep love and appreciation of the natural world - many anglers considered "great" have had this, and also had an ability to communicate this to others, either in words or verbally (Yates, Isaac Walton etc.).
2. Enthusiasm - of course, to be a great angler you must be passionate about fishing. John Wilson (love or hate him) has this in abundance, and is probably responsible for more people of recent generations taking up angling than any other person.
3. Knowledge - Goes without saying that a good angler has extensive knowledge, but a great one must add something of his own to this, instead of merely standing on the shoulders of giants. I.e. Richard Walker devising new ways and methods to improve his chances (lot of "science" there, Anderoo).
4. Good old fashioned luck - Is there any such thing as luck in angling? (Good subject for a post of its own, maybe?). Sometimes, when I catch what I want, when I want, I attribute it to "skill", "knowledge" or "sixth-sense", but there is a large elememt of chance involved. When Yates caught his record carp from Redmire he claimed he somehow "knew" the fish was there. True, or just a way of justifying a lucky capture after the event?