What's so mysterious about consciousness?

Here's an off-the-cuff response I gave to somebody on YouTube today who didn't think consciousness was all that mysterious.

I think the mystery for physicalists isn't in THAT consciousness arrises out of a physical substrate, but in HOW it does so. Consciousness is different than everything else in the physical world in a fundamental way. Everything else in the physical world is describable, at least in principle, in third person terms. If the properties of some physical object are observable to one person, then they're observable to anybody else who has the same instruments with which to do their observations.

But consciousness is different. The only person who can feel your feelings, see your perceptions, or experience what it's like to be you is YOU. Take something as simple as a visual perception. When you're dreaming, for example, you can very clearly see visual images of people, plants, and all kinds of things, but if you looked into the brain while the person was dreaming, you wouldn't see any of those things. And it doesn't even make sense to say that you could "see" an emotion or a desire or an intention.

To me, that's a big part of what makes consciousness so mysterious. But the biggest mystery to me is that supposedly our conscious states have some causal influence over our behavior. It's a mystery how a desire or a motive can result in me moving my arms and legs. We can explain this in terms of the third person physical properties of our neurons, but how do you explain it in terms of the semantic content of the desire, belief, or motive?

We can conceive of equations to describe the movement and behavior of physical things in terms of their third person properties, but how can you conceive of an equation to describe the "what it's like" of first person subjective experiences? Isn't that mysterious?