This is what the late Mr. Zelazny had the most of: style. Most of his books are enjoyable just because they are so well written. They don't always have the greatest concepts, and rarely do they have the latest and greatest in techno-wizardry. But could that guy turn a phrase! Just an amazing wordsmith. His stories stand up to a re-read possibly better than anybody else.
What he writes is generally fantasy/philosophy with science underpinnings. It is hard to categorize. It's just plain good.
It's not easy to say why. Perhaps it is more subjective than I would like to believe. He often wanders into philosophical alleys and side streets which probably drives some people crazy who just want to follow a plot line.
For example, one of his stories is about fishing on Venus: The Doors of his Face, The Lamps of His Mouth. Now even at the time, everybody knew the environment of Venus was nothing like in the story. And the idea of ocean-going Leviathans as depicted was absurd. But this did not detract from the story! Which I believe won Hugos, Nebulae, and every other award in sight.
His best-known blockbuster is probably Lord of Light. Somewhat less sweeping in scope, but very engaging are This Immortal and Isle of the Dead. He wrote some good short stories, as in The Unicorn Variations and The Last Defender of Camelot. (This cracks me up. In an intro to the title story in the latter book, he says he sent the story to the Saturday Evening Post, who wanted him to cut the length in half. He said he tried taking out every other word, but it sounded funny, so he decided to leave it alone!)