According to Leslie Valiant
The most critical choice for a scientist is what problems to work on. The successful ones find problems of significance on which progress is possible using their particular skills.
According to Michael Kearns
One strongly feels that his taste in problems is inherently personal. He picks very big and fundamental questions and pursues them quietly and rigorously until he has something important to say, and only then will you find him writing a usually landmark paper. In this sense he is immune to fashion, more so than any researcher I have ever encountered.