There is muc to be contrasted between the cultures of the Harappans and the Aryans. There are indeed a few points of similarities, but they are not of any significance. Why the points of contrast are more is primarily because of geographic location, economic activity and the religious practices followed by both the cultures. Far more important is the fact that the Aryans, with a plasticity of mind, made life vibrant; whereas, the Indus life looks more like stylized puppet show.
The plasticity of the Aryan mind was shown in the language as well as the way in which they adapted agricultural and settled life. The seals of the Indus Valley show that the pictographs remained statis, whereas, the Aryan language in the Rig Veda at places rises to musical levels. The success with which the Aryan writings were composed reveals the ability of the Aryan mind to grasp the mulitiple dimensions of human life. And language which exhibits immense potentialities in its vocabulary reveals that the community is full of potentialities. On the other hand, out of nearly 400 characters known to the Harappans only a few were repeated time and again.
The other manifestation of Aryan civilization, that is, its capacity to change and adapt itself, has given a continuity to Indian Civilization despite the absence of mighty empires. On the other hand, the Indus Valley people reached a blind alley and the never learnt anything from other civilizations like the Sumerian. Adaptability or ability to respond to challenges is the hallmark of any youthful civilization. The Indus civilization reached its senilithy by 2000 B.C. whereas the Aryan Civilization was full with creative dynamism.
Archaeology is the only source of our knowledge of the Harappan civilization, but information concerning the Vedic Aryans depends almost entirely on literary texts, which were handed down by the oral tradition. It is clear from the material remains that the Harappan civilization was in certain respects superior to that of the Aryans. In Particular it was a city civilization of a highly developed type, while by contrast city life was unfamiliar to the Aryans. The superiority of the Aryans lay in the military field. In which their use of the light horse chariot played a prominent part, or in literary exuberation.
Harappans were peace loving city-dwellers and good planners as is evident by grid pattern towns, elaborate drainage system, street lights, kelp-burnt brick houses, fortifications, granaries, baths and wells. The early Aryans were not city builders. Their way of life, nomad-pastoralists as theywere, was dominated by war like stock-breeding (they practiced a little agriculture) and migrations. City buildings etc. as a large-scale socio-economic activities is only much later mentioned in the later Vedic texts, epics and the Puranas.
The Harrapa culture is located in the Indus Valley and western India and its urbanization is based on a chalcolithic system with and absence of iron. Later Vedic society centering on the Ganges Valley from which the Harappan culture is largely absent owes its gradual urbanization to iron technology, the widespread domestication of the horse and the extension and intensification of plough agriculture. (Iron, horse and plough being nearly absent - some evidence in later Harappan sites).
The expansion and budding off of the Harappan system in the east as far as Alamgirpur (U.P.) and to the neighbouring areas was neither 'colonisation' nor was it 'political expansion' of any from, it was rather the expansion in terms of the permeations of the socio-economic and socio-cultural systems of Harappan society whereas, the Aryan advance towards eastern region - the Doab of the Ganges and Jamuna - was no doubt facilitated by their horse chariots and effective weapons and can be viewed as 'colonisation' or 'political expansion' though not all the Aryan culture contacts and expansion need have been of a violent kind.
The focal centers of the Harappan culture remained for a long time the twin cities of Harappa and Mohenjo-daro and it is from these centers that Harappan culture budded off, whereas the focus of attention of the Rig Veda was the Punjab and in the later Vedic period it shifted to the Doab of the Ganges and Jamuna rivers. The Punjab seems gradually to fade into the background and was regarded even with disapproval.
The Harappan society had a very complex social stratification, division of labour and multiplicity of crafts and industries, urbanism was its marked feature with Harappans enjoying a settled and sedentary life, and in this society the priest and the merchant played dominant roles perhaps constituting a 'ruling' elite. On the other hand, in the early period the Aryans were organized into a social organization which may be described as 'tribal' or rural' one with a minimal of division of labour and sedentariness. It was sed fully with more pronounced and increased division of labour when specialized trades and crafts appeared. But in this society it was not the priests and the merchants (Vaishyas) but the Priests and the Kshtriya who constituted the rule in elite (though with a tendency to rivalry).
In the Harappan society the Priestly class was of great importance as the central authority. Though there is little evidence in the Rig Veda of any special importance of the priests, however in later Vedic society, the priests as a class assumed a form of institutional authority. The institutions of slavery and prostitution were common to both the societies.
The entire Harappan civilization was the product of an available food surplus (wheat and barley), a fairly high level of craft industry, a script and most important of active commercial intercourse by which it was able to obtain its different and varied material from places far and near both in India (the sub-continent outside the Harappan sphere was not terra-incognita) and outside (i.e. Sumerian towns, Baluchistan and Central Asia). Both northern and southern India was connected in Harappan period by ties of brisk trade. But the early Aryans did not fully emerged out from the food-gathering and nomadic pastoral stage. They hated the panis, i.e. those who indulged in trade. Though by the end of the Vedic age trade contracts and commercial inter-course did not reach the Harappan level. It was only by the end of the Vedic period that the Aryans had some familiarity with the sub-continent.
The religion of the Harappan differed widely from that of the Vedic people. The Harappan practiced the cults of Sakti (mother Goddess) and Pasupati (Proto-Shiva) of animal-tree and stone worship and of Phallus and Yoni, i.e. fertility cult. The early Aryans condemned many of these cults. Harappans worshiped Mother Goddess but the Female deities played a minor part in Vedic religion though the Aryans provided spouses to their gods by later Vedic times. But the fear of the Phallus worship was replaced in the Yajur veda by its recognition as an official ritual. Siva also gained increased importance in the later Vedas. The Aryans anthropomorphized most of the forces of nature and prayed to them as Indra, Varuna, Agni, Mitra, Rudra, Soma, Surya, and Asvins. The fire of sacrificial cult was common to both. Vedic Aryans worshipped the cow while the Harappans reserved their veneration for bulls. The Harappans were iconic and the Aryans aniconic. Ascetic practices were known to both.
That the Harappan had a ruling authority or elite and / or an administrative organization cannot be doubted. Almost uniform planning of the cities and presence of sanitary system, standard weights and measures, assembly halls, huge granaries and citadels point to the existence of an authority, but what it was like as the later Vedic period the Aryan tribes had consolidated in little kingdoms with capitals and a sedimentary administrative system with important functionaries the Purohit and the twelve ratrins playing dominant role in support of the monarchy, the prevalent form of government.
The food habits of the Harappans were almost identical with those of the later Aryans if not early Aryans. The Harappans unlike the Aryans, preferred indoor games of outdoor amusements (chariot racing and hunting) though dice was popular past time with both. Playing music, singing and dancing were common to both. But about the musical instrument of the Harappan little is known or not known while the Aryans had the drum, lute and flute with cymbals and the harp as later additions. The Harappans buried their dead - the Aryans largely created their dead. The Harappans used a script, which remains undeciphered to date in spite of many claims for its deco din, where as references to writing in Vedic society came at a much later stage.
In art the Harappans made considerable progress. Their works of art add tour comprehension of their culture. In fact, the earliest artistic traditions belong to them. In sculpture (beareded man from Mohenjo-daro and two sand stone statuettes from Harappa), though a very few sculptures survive, in metal (bronze dancing girl) and ivory works, in terracotta's (small images and figures of animals, birds or human or animal and inscription a 9 Harappan script on them), and in their pottery (painted red and black, at times glazed), the Harappan show vigor, variety and ingenuity. On the other hand, Rig Vedic age is devoid of any tangible proof of Aryan achievements in these directions. In fact the Rig Veda says nothing of writing, art and architecture. The art of ceramics made Harappan, the Vedic pottery was a simple one.
The Harappans lacked that plasticity and dynamism of mind which is very essential for further growth and survival and they refused to learn from others, on the other hand, the Aryans possessing what the Harappans lacked, were youthful enough to be receptive, adaptive and assimilative, transforming themselves into a comprehensive civilization which in due course of time became essentially composite in character.
In the end we have to say that apart from the minor causative factors causing difference like the close mindedness of the Harappans and contrasted to the Plasticity of the Aryan mind, formalized and ritualized religion of the Harappans as contrasted to the animals and the metaphysical traits of the Aryans and the geographical locale were entirely different. The differences in socio-economic matrices between the two civilizations primarily account for the contrast between the two.