Marx stressed alienation from one's work, from the product of one's labor, and from human nature. He applied the term ‘alienation’ to man's loss of independence, his impoverishment, and his estrangement from his fellow men; but above all to man's condemnation to labor that is devoid of all originality, spontaneity, and creativity. The last point was much the most important to his mind. Creativity was for him of the essence of man, and he considered man's alienation from that the root evil from which all the other evils were derived. The original sin was the dehumanization of man through the disappropriation of his creative power.
The production of too many useful things results in too many useless people.
—Marx, Economic and Philosophical Manuscripts
[T]he more the worker expends himself in work, the more powerful becomes the world of objects which he creates in face of himself, the poorer he becomes in his inner life and the less he belongs to himself; it is just the same as in religion. The more of himself man attributes to God the less he has left in himself. The worker puts his life into the object and his life then belongs no longer to himself but to the object. The greater his activity, therefore, the less he possesses.
—Marx, Economic and Philosophical Manuscripts
In handicraft [...] the workman makes use of a tool; in the factory the machine makes use of him. There the movements of the instruments of labor proceed from him; here it is the movement of the machines that he must follow. In [handicraft], the workmen are part of a living mechanism; in the factory, we have a lifeless mechnanism, independent of the workman, who becomes its mere living appendage."
—Marx, Capital
As long as a cleavage exists between the particular and the common interest man's own deed becomes an alien power opposed to him, which enslaves him instead of being controlled by him. [...] This crystallization of social activity, this consolidation of what we ourselves produce into an objective power above us, growing out of our control, thwarting our expectations, bringing to naught our calculations, is one of the chief factors in historical development up to now.
—Marx, The German Ideology
Communism is the positive abolition of private property, of human self-alienation, and thus the real appropriation of human nature through and for man. It is, therefore, the return of man to himself as a social, i.e., really human being, a complete and conscious return which assimilates all the wealth of previous development.
—Marx, Economic and Philosophical Manuscripts
Marx wrote some interesting comments on “crude communism” which serve to show how opposed his thinking was to what now calls itself the “left.”
[This crude communism] appears in a double form; the domination of material property looms so large that it aims to destroy everything which is incapable of being possessed by everyone as private property. It wishes to eliminate talent, etc., by force. Immediate physical possession seems to it the unique goal of life and existence. The role of worker is not abolished but is extended to all men. The relation of private property remains the relation of the community to the world of things. Finally, this tendency to oppose general private property to private property is expressed in an animal form; marriage (which is incontestably a form of exclusive private property) is contrasted with the community of women, in which women become communal and common property. One may say that this idea of the community of women is the open secret of this entirely crude and unreflective communism. Just as women are to pass from marriage to universal prostitution, so the whole world of wealth (i.e., the objective being of man) is to pass to the relation of universal prostitution with the community. This communism, which negates the personality of man in every sphere, is the only logical expression of private property, which is this negation. Universal envy setting itself up as a power is only a camouflaged form of cupidity which reestablishes itself and satisfies itself in a different way. The thoughts of every individual private property are at least directed against any wealthier private property, in the form of envy and the desire to reduce everything to a common level; so that this envy and levelling in fact constitute the essence of competition. Crude communism is only the culmination of such envy and levelling-down on the basis of a preconceived minimum. How little the abolition of private property represents a genuine appropriation is shown by the abstract negation of the whole world of culture and civilization, and the regression to the unnatural simplicity of the poor and wantless individual who has not only surpassed private property but has not yet even attained to it. The community is only a community of work and of equality of wages paid out by the communal capital, by the community as universal capitalist. The two sides of the relation are raised to a supposed universality; labor as a condition in which everyone is placed, and capital as the acknowledged universality and power of the community.
—Marx, Economic and Philosophical Manuscripts
Marx considered communism to be an “end of history” — a stable state of society in which no social conflict (or at least "class war") would remain. Under communism, Marx believed that the whole of ‘man’ would be united in the collective conquest of nature. He believed, moreover, that industrial technology would provide an unlimited power to shape the material world, and thus the world of human life. Thus through the elimination of conflict in the organization of production, ‘man’ could design his society (put otherwise: society could be designed to satisfy human needs). C.f. Fromm on Marx.
Communism as a fully developed naturalism is humanism and, as a fully developed humanism, is naturalism. It is the definitive resolution of the antagonism between man and nature, and between man and man. It is the true solution of the conflict between existence and essence, between objectification and self-affirmation, between freedom and necessity, between individual and species. It is the solution of the riddle of history and knows itself to be this solution.
—Marx, Economic and Philosophical Manuscripts