I think there are other variables that unite man. I haven't had an inclination to the conclusion that hope is what unites man. I have been in and out of mental institutions my whole life, and hope there can be very lacking. These people have gone through wars through major loses, grief stricken, struggling with hardship. Hope by itself with no defined consequence (general as it would have to be) is rather obscure, and to conclude that hope itself is unifying would seem false to me personally.

Not sure how to understand what is being asked.

Discord, dissension, strife, conflict might just as well be causes of temporary unity. To identify and sacrifice for an identity by sharing attributes, hardships, memories, experiences, culture, desires, beliefs et cetera, is as much cause of unity as disunity. A union requires existential conditions for its unity, whether emerging from natural states (self-organizing structures) or careful human deliberation.

Eating and drinking is a universal necessity among a lot of species. So maybe the recognition of this fact of survival has the potential to unify. But the belief as to what one ought to eat may separate us as much as it unifies.

That which by belief requires a conservation of its union must work to protect that union.

Life unifies by dissembling itself. Life eats life.

Fear also creates unity.

Fear also creates unity. — Waya

Lao Tzu says:

Success is as dangerous as failure.
Hope is as hollow as fear.

What does it mean that success is a dangerous as failure?
Whether you go up the ladder or down it,
you position is shaky.
When you stand with your two feet on the ground,
you will always keep your balance.

What does it mean that hope is as hollow as fear?
Hope and fear are both phantoms
that arise from thinking of the self.
When we don't see the self as self,
what do we have to fear?

See the world as your self.
Have faith in the way things are.
Love the world as your self;
then you can care for all things.

This is consistent with my own experience of both fear and hope. It ties in with another recent thread - The Source of Suffering is Desire? Hope, fear, desire - They push us, pull us, lead us to live in a different time and place than where and when we are.

So, if that's true, what would it mean if hope and fear are what all people have in common?

Common values and goals creates unity. More the latter, in some cases where, for instance, the enemy of my enemy is my friend.

Your view of humanity is interesting, but I suppose what really unites us is common ground. It doesn't matter whether you have hope, or love even, when you can find common ground. Examples would be the people who hate every one, their common ground would be hate, and everybody hates something. Belief, religion and favourite items can all unite us, the problem is people disagree on some things, but they will always have something in common.

The common narratives of being unite humanity. Obviously we have great difficulty in communicating our perspectives of this narrative though! :D

What actually unites mankind?

What we're made of.

What unites us? The fact that we're the same species, and we're a species that survives more easily when we interact with/cooperate with each other.

How about these obvious items: language, art, music, food and mortality?

Didn't Gandhi once say "There are enough religions to divide us but not enough to unite us"? I think this points to only one thing - that we haven't found unity even in our best form - morality, to speak nothing of other areas where division, rather than unity, is the norm.

If you want my opinion though, the one thing that unites us all is our mind - the capacity for abstract thought. Didn't the "great" Aristotle define man as the rational animal?

Technology. Technology unites man because everytime it is used, or thought of, we are reminded that human cooperation took place to create the technology. Why not language, art, music, food and mortality? Because technology has greater potential for work. Then why not work? Why can't we say work unites man? Work is not an absolute unifying concept because it is rarely thought of during leisure and survival, and has been throughout history a reason to separate.

Why can we not say more absolute concepts than technology are the reason for unifying man? Because technology is tangible and common.