At a job interview some time back, the managing director of the company I was interviewed by named two different WCM systems. Then he asked me which one of two I believed was the best. He obviously wanted to test my expertise in WCM, which he had heard of before the interview.
Although I was quite familiar with both products, I answered him that I couldn't possibly pick one or the other. Not without first understanding the needs of the organization that was to choose between the two products. Using my expertise in web content management, I could help them identify and describe their needs, state them as requirements, map those to the technical capabilities of each product, and then recommend the product that best fits their requirements (needs). Doing so requires you to ask the right questions, and knowing how to find the answers.
I believe it wasn't the answer he had expected, but I do hope he learned something about what it means to be an expert from my answer.
Being an expert is not about knowing everything within a certain domain. It is about being able to ask the right questions, and having the skills and network to find the answers.
In fact, if I hadn't been connected to so many smart people myself, I would have missed adding network part of being an expert (thank you @KluwerLearning).