one thing i do really see as running throughout the history of the left is an enthusiasm for collective consciousness. the idea of forging such a consciousness politically goes back at least to hobbes, though, in the modern period. but rousseau's 'general will' adds an inspiring twist and starts to frame the thing in terms of social justice. now, i am going to say that we are not fully distinct from one another, nor from the rest of the world. but the applications of this thought have been nightmarish. even in rousseau, the collective consciousness is forged by subordination (dissenters will be 'forced to be free').
we have an impulse to merge, to lose ourselves, to be at once expanded and erased. perhaps it is because we are sexual creatures. but truly, the idea is that if everyone could be fully subordinated by one set of rules or rulers - with the materiel to make it stick - we would finally be as one. we wouldn't be lonely anymore.
plato, in setting out his ideal polis, based it on the idea that the polis is analagous to the human self (it is divided into the same three parts, etc.). the dream of unity is old, so are the mechanisms used to simulate it (plato likes lies and eugenics, which is fairly par for the course).
i think we should agree on things and we should act in concert if the action is good. but we are also stuck with our irremediable distinction from one another, and any of us can dissent from any consensus, which is one reason we're not all one self/will/belief system together. and the consensus is very likely to be wrong, and very likely to be merely enforced.
it is not only the left, though, that likes or worships a merger of many into one. the very idea of the corporation as a legal person enshrines collective consciousness in law. hegel was as enthusiastic about the idea as marx, and called The Thing the nation or state. mussolini liked it as much as did pol pot.
in every candidate for collective identity - nation, class, race, corporation - listen to the high-flown rhetoric, and then watch the actual coercion and force by which the collective action and identity is actually achieved. what is supposed to be an antecedent reality that we're detecting (our 'social nature' etc.) is a fiction brought to a leering semblance of reality by violence.
what i hear when someone brings in the collective or the people or whatever: you have to agree with me, because you are me. so if you disagree with me you betray yourself. thus, by your own fiat i will constrain you: the whole idea calls you a betrayer of yourself in order to attack you. this is what modern political philosophy often means, bizarrely enough, by autonomy, citizenship, democracy and so on. it's what habermas means, for example.