Subjective conscious experience. The discussion of qualia tends to come up on LessWrong in two contexts: as an argument against reductionism (with a claim that qualia cannot be a mere matter of, uh, matter), and as a key factor in how seriously we should weight the suffering of animals. There is also a third context for Qualia arguments: as an argument against Whole Brain Emulation.
There are several philosophical discussions of how to reduce qualia into functions of the brain (most famously by Daniel Dennett), but the discussion continues among philosophers.