A colleague that I really respect told me at my last job that his career was a series of broadening and deepenings. He started with a deep knowledge of Java, but found that success came as he branched out into Python, Ruby, and Scala. At that point, he discovered that developing a focus on distributed computing with Scala was the key to the next level, and so on. I think half the challenge is identifying when its time to do one of those two.