As far as in the ring, I think one of the things that makes a great wrestler is how they work with their opponents. If you watch DB, you'll see that he sells shots well and can make the other person really step up their game. Same is for guys like Jericho and even Punk.
What you're looking for is chemistry in the ring between the two opponents. If you get two guys who don't sell very well and whatnot, it makes for a pretty crap match (Khali is a great example of someone who just really sits there and does nothing). A great wrestler should bring out the best out of their opponent and really make the match amazing to watch.
It's one of the reasons people have a problem with Cena, he doesn't sell very well and he's got an extremely limited move set that makes for the same match every time. A great wrestler should be able to have a different match every night and do something different.
If you haven't watched the movie Breaking the Code, The Chris Jericho story, I would recommend doing so. It's on Netflix. He talks about what he learned as a wrestler in different places all over the world and how he brings that into his wrestling.
That's just my thoughts.