It seems to me that the notion of expertise can only apply to fields in which there is an established body of knowledge. By that I mean fields in which we have (empirical) grounds for believing our knowledge is at least an approximation or heading in the right direction. Physics or genetics or how to fix cars are examples of such fields. You can be an expert in physics.
Philosophy seems different. What makes philosophy interesting is that it's about things we don't understand well. In philosophy we're not even sure that existing approaches to problems are heading in the right direction.
Philosophy is pretty much by definition about things we don't understand well. Once a philosophical topic is understood it ceases to be part of philosophy, and becomes part of another field like physics, biology, economics, etc. (or alternatively, the problem may be dissolved and seen as a kind of misunderstanding.)
I would say the kind of knowledge that exists in the field of philosophy is more of ways of describing problems, or particular arguments for or against a view of problems. It's more like a discussion.
You can be an expert in the different positions about a philosophical problem, but I would distinguish this from the idea that someone can be an expert on a philosophical subject.
For example, someone can be an expert on the various problems and arguments associated with consciousness, but I don't think anyone can claim to be an expert on consciousness (at least the hard problem of consciousness) because we just don't understand it.
So rather than saying there are experts in philosophy I would say that there are people who are very learned in philosophy.
Why does this distinction matter?
When there isn't established knowledge, we're less certain that existing approaches are correct. The fact that an existing approach hasn't been able to solve a problem for long time may mean that it's the wrong approach. It is more likely in philosophy that someone who comes from outside of the field, who isn't well versed in the existing approaches, can add something of use to the table. The fact that they aren't familiar with existing arguments may even be a virtue.
If there aren't philosophical experts, then there aren't experts to challenge.
Yet it seems to me that philosophy seems to hold greater reverence for 'experts' than most other fields.
What do you think?
[I originally posted this to reddit/r/philosophy but -- to my surprise, since it is somewhat critical of how philosophy is done -- it didn't generate any comments.]