(With a short discussion about some other questions, as well.)
Consciousness is sensation, qualia, experience itself. Pain is not the flinch, the transmission of nerve signals, the yelling of 'ow.' Pain is the (sensation, qualia, experience) of suffering, which may lead to the flinch, or may lead from the flinch, or may be parallel to the flinch.
The first is our general experience of experience; we feel pain and so we flinch. The second is a reinterpretation of the physical state; that experiences are meters that report what the body is doing; the particular pattern of flinches, hormones, and nerve signals gives rise to the report of suffering. The final is something like a branch network; the nerve signal reaches the brain which releases hormones and the 'ow' independently of producing the suffering. The flinch and the suffering have roughly the same cause.
All objective facts must be verified by inferring the causes behind subjective experiences. For instance, to verify the temperature, I have to infer the cause behind a particular arrangement of red, the distortion made by the glass, (inferring the glass and distortion as well) and the black graduations (inferring the shape of the symbols and associating them with numbers.)
By contrast, there is no need to verify red or suffering. They simply exist, in the most first-hand way possible. Logically speaking, the existence of experience is axiomatic. Sensations are the most fundamental unit of logic.
The people who say there is no problem of consciousness - that consciousness doesn't exist - are willfully confusing the bare sensations with all the attributes and purposes that have glommed onto them over the millennia - willful choice, identity, morality, dualism, and so on - which are indeed debatable.
Indeed every property of consciousness aside from its existence is sublimely dubious, which is why you can claim that it doesn't exist without instantly losing all self-respect. It is still incoherent to say that experience doesn't exist, because every moment I am awake is a counter-example. (You're still allowed to claim that you don't exist, because of the other minds problem. If you do, I'm taking all your stuff, though. It's not illegal to steal from buildings - they cannot suffer from the act, and if you're not conscious, neither can you.)
The other confusion is over the question, "Can a thing be conscious without acting conscious? Could my chair have some internal experience, despite the fact that it will never reflect this fact?"
The first problem for this is the problem of measurement. For the first, even if I am conscious, without eyes, ears, and so on, I would be unaware of the outside world, incapable of, for instance, suffering due to attack. I may dream, but it would be impossible to awaken. Thus, even if chairs are conscious, we cannot abuse them. This also suggests that there needs to be some kind of computation to interpret the data from the sensors,* which most likely rules out consciousness for plants. While they react to the environment through some kind of sensory apparatus, there is no CPU to interpret the data.
*(The existence of dreams also handily rules out the consciousness of the sensors themselves. Interpretation can occur without actual input.)
There's also a similar problem with consciousness' effect on the outside world, (for instance, the chair has no muscles and even then wouldn't be able to move them) a property I like to symbolize with 'choice.'
The second problem is the problem of locked-in humans. They see and move much as a chair does - not at all. However, they still send information to the outside world, which we can pick up with fMRI and in principle verify that they are dreaming. (As usual, still subject to the other minds problem; this verification uses correlation purely.)
So yes, consciousness exists. The argument begins just beyond.
Is there anything I've missed?
UPDATE: No, there isn't. To present to me the argument that I am not conscious requires that I become conscious of your argument. The argument is the falsification.