They have their own style/voice. When one reads a sentence or a paragraph constructed by Kafka or Barthelme or Beckett, he/she knows almost right away who the writer is, just like when one hears The Police on the radio.
To bear witness to this phenomenon, one should consider the following paragraph from Barthelme's short story, "Indian Uprising."
"The girls of my quarter wore long blue mufflers that reached to their knees. Sometimes the girls hid Comanches in their rooms, the blue mufflers together in a room creating a great blue fog. Block opened the door. He was carrying weapons, flowers, loaves of bread. And he was friendly, kind, enthusiastic, so I related a little of the history of torture, reviewing the technical literature quoting the best modern sources, French, German, and American, and point out the flies which had gathered in anticipation of some new, cool color."
This is a very discursive paragraph that makes little sense when removed from the context of the larger narrative. However, it is vintage Barthelme. There is a frenzied quality to this paragraph that is entertaining. There is a sudden rush from one subject, the girls and their mufflers, to the next, the man named "Block."
As one reads it though as a self-contained piece, the paragraph exhibits the humor and contradictory feel that characterizes Barthelme's prose. A man named "Block" opens the door. That's funny because of the obvious contradiction between the words "block and open." He brings with him both weapons and flowers. That's also very funny because of the creative juxtaposition of symbols of both love and war. And, of course, the fact that the narrator shares with Block a little of the history of torture, undoubtedly a disgusting subject, but one that is made humorous by Barthelme's choice to point out that the narrator reviewed "technical literature and the best modern sources" to instruct Block about torture. In this way, Bartheleme de-emphasizes the serious nature of the subject and creates a more playful and humorous tone and style.
Then, at the end of the paragraph he ties it all together with the comment about the flies waiting for "some new, cool color." Color, if one will recall, was used at the beginning to remark on the color of the girls' mufflers. The point is only Bartheleme could have written that paragraph. It has all the elements of his style, humor, contradiction, play on words, discursiveness, a frenzied pace, esotericism. Like all great writers Bartheleme has his own voice, his own style.
The last aspect that makes great writers great, in my opinion, is their attention to detail. As Nabokov once said in his essay "Good Readers and Good Writers," -- "In reading, one should notice and fondle the details." The details are important. In the end, they separate the boys from the men. For example, consider the use of dynamic details in the short story, "A&P" by John Updike, "The one that caught my eye first was the one in the plaid green two-piece. She was a chunky kid, with a good tan and a sweet broad soft-looking can with those two crescents of white just under it, where the sun never seems to hit, at the top of the backs of her legs. I stood there with my hand on a box of HiHo crackers trying to remember if I rang it up or not."
This is a great string of sentences filled with eye-catching detail. Consider if those sentences were written as follows, "I saw the girl in the plaid bathing suit. She was over-weight with a tan butt that was white at the base. I stood there with my on some crackers." This rendition is rather uninspiring and sloppy. The "two crescents" of white flesh is such a wonderfully accurate image. It's a subtle point, but it's one that most readers can identify with. Moreover, it's not a box of crackers, but HiHo crackers. This little detail helps to bring the reader right into the drama of the story. In many ways, Updike is a champ at adding details (sometimes Updike adds too many; but this is a discussion for another time).
There are four aspects to writing that I really admire, those are simple beginnings, complex and layered storylines, unique styles, and dynamic details.