It seems clear to me that most common definitions of consciousness do not lend themselves to precise categorization. There's a slope from bacteria to the nematode worm to amphibians to mammals and birds to primates and cetaceans, and then humans on top, because we're clearly better at this than anybody else, for the time being. You can impose a decision boundary on that slope, but that doesn't tell you anything about the real world.
The Francis Crick Memorial Conference, held in Cambridge last month, has come up with the Cambridge Declaration on Consciousness (PDF).
tl;dr humans still aren't special, consciousness seems to arise in quite a variety of nervous systems and working out what it is is a problem in neurology.
We declare the following: “The absence of a neocortex does not appear to preclude an organism from
experiencing affective states. Convergent evidence indicates that non-human animals have the
neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with
the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that
humans are not unique in possessing the neurological substrates that generate consciousness. Non-
human animals, including all mammals and birds, and many other creatures, including octopuses, also
possess these neurological substrates.”