All Hemingway's writing is pithy and "economical", totally unlike the flowery writing with complex sentence structures that had come before. I don't know that he thought he was a great writer -- he thought Flaubert was the best, and Flaubert's style is unlike his.
I think what made Hemingway popular was his "modernness". He wrote about sex, psychological issues, and heavy drinking which was quite the contrast to the Victorian writers.
My favorite of his has always been "The Sun Also Rises" which he wrote when only twenty-six. I'm not even sure he understood the underlying sexual and psychological issues which he portrayed there.
