>what do you think is so special about humans?

The ability to argue such things, the interest in doing so, and the capacity to benefit from the activity. (These are all the same thing, BTW.)

>And how do you take into account the gulf between a human with severe cognitive deficit vs Albert Einstein level of cognition?

What account needs to be taken? Are you suggesting that we should consider people who aren't smart to be less human? I dispute that there is anything such thing as a "level of cognition", merely an apparent difference in the results of cognition based on the circumstances. You can either accept that all humans are conscious because they are humans and consciousness is endemic (not guaranteed but probable) in humans, or you can insist that some humans are less human because of what you describe as a "cognitive deficit" of some arbitrary "level" of severity. I take the simpler approach, and stick with considering consciousness as either a categorical property, or an instance of behavior, as appropriate for the context, without falsely assuming all contexts must be considered identical in this regard. For example, it isn't really as confusing as you might believe to say that a sleeping ("unconscious") Einstein is not smarter than a developmentally delayed child who is awake, yet both are equally conscious creatures and fully human.

>What bare minimum do humans have that other animals lack?

The scientific phrase would be "neural correlates of consciousness". The philosophical term would be "mind". AKA "reasoning".

Deal with it. 😉