"The castes were hardened by the time the Aryans occupied the middle land i.e., the Gangetic Valley and distinguished themselves from their brethern in Sind and the Punjab who were despised by them for not observing the rules of caste .... and for their non-Brahmanical character.” (Sindhi Culture, By U.T. Thakur)
“While some Aryans had by now expanded far into India, their old home in the Punjab, Sind and the north-west was practically forgotten. Later Vedic literature mentions it rarely, and then usually with disparagement and contempt, as an impure land where sacrifices are not performed.” (The Wonder that was India, By A.L. Bhasham)
This is further supported by Dr. Gurupdesh Singh:
"From geographical information in the RigVeda, the Vedic Period (1500-500 BC) was confined to the northwest. The hymns composed by Vedic mystics/poets of the northwest (Saptha Sindhva) tell that the Vedic peoples worshipped non-Brahmanical Gods (Indra, Varuna, Mitra), ate cows, elected their chiefs, drank liquor, considered the Punjab rivers to be sacred, and refer to people living to the south in the gangetic region as 'Dasyas'! None of the Gangetic Brahmanical gods (e.g Ram, Krishna, Vishnu, Brahma, etc.) are mentioned in RigVeda hyms nor do they appear in connected Aryan Avestan texts and Hittite tablets. Avestan terms for soldiers ('rathaestar') and citizens ('vastriyo') are similar to Vedic-derived terms (kshatriyas, vasihyas) but the Avestan term for priest ('athravan') is not even close to 'Brahmanas'. Moreover, central Gangetic religious texts like the Mahabharta and Varna Ashram Dharma of Manu call the Vedic Aryans in Saptha Sindhva 'mlechas', 'sudras' and 'vratyas'; 'forbid Brahmins' from even visiting the northwest country ('Vahika-desa'); and depict dark Dravidian Gods like Krishna fighting and defeating Vedic Aryan gods like Indra (Mahabharta). Similarly, the RigVeda contains taboos and injunctions against the 'dasya-varta' region to the south of Saptha Sindhva and praises Indra (god of thunderbolt) for victories over 'dasya-purahs' (dasya cities).
"Both early RigVedic and Gangetic Puranic sources clearly point to ethnic, cultural and religious differences and a 'clash of civilizations and nations' at the ganga indicating that the Vedic people and culture of the northwest did not accept the Gangetic priests, their gods, shastras, religion, culture and Brahmanical caste ideology. The eastern gangetic heartland is not only historically a separate region, but geographically resides over 1500 miles to the southeast of the Saptha Sindhva country. Uptil the advent of Mohammed Ghori in the 13th century, the northwest was politically unified with southasia only 92 years under the Mauryas (out of 27 centuries) since the start of Saptha Sindhva’s Vedic period (1500 BC).
"A few Vedic tribes from Saptha Sindhva broke RigVedic norms and migrated southward. These numerically outnumbered groups expanding into the trans-gangetic region near the end of the Vedic period (8-6th century BC) tried to use the indigenous Dravidian priesthood to entrench themselves as the new ruling order. Within a few generations of acquiring control over the foreign Gangasthan, the minority Vedic tribes were usurped by the indigenous 'borrowed' priesthood; their Aryan religion, gods and customs mostly deposed and supplanted with indigenous gangetic gods and mythologies; and their new social order (varna or color based) replaced with the pre-existing profession (jati) based Brahmanical caste system ('chatur-varna' ). Through religious manipulation and intrigue, the Vedic in-corners to Gangasthan were usurped and made to surrender their political rule and soon pigeon-holed into becoming the loyal obedient chownkidars of their 'superior' dravidic Brahmanas."
Now coming to idolatry which is an integral part of Hinduism, there are clear evidences of early Vedic Aryans rejecting it :
“They are enveloped in darkness, in other words, are steeped in ignorance and sunk in the greatest depths of misery who worship the uncreated, eternal prakrti—the material cause of the world—in place of the All-pervading God, but those who worship visible things born of the Prakrti, such as the earth, trees, bodies (human and the like) in place of God are enveloped in still greater darkness, in other words, they are extremely foolish, fall into an awful hell of pain and sorrow, and suffer terribly for a long time.”—Yajur Veda 40:9.
“The Formless Supreme Spirit that pervades the universe can have no material representation, likeness or image.”—Yajur Veda 32:3.
Also, early Vedic Aryans had a Monist belief of worshipping elements of nature (in non-idolatrous personified forms): “There is only one God, worship Him” (Rig Veda, Vol. 6, Hymn 45 vs 16 ) and “Do not worship any one beside Him” (Rig Veda Bk. 8, Hymn 1, Vs 1)
Then there are clear evidences in the Rig Veda that Aryans regularly ate beef and sacrificed cows for religious purposes which are strictly forbidden in Hinduism:
Hymn CLXIX of the Rig Veda says: "May the wind blow upon our cows with healing; may they eat herbage ... Like-colored various-hued or single- colored whose names through sacrifice are known to Agni, Whom the Angirases produced by Ferbvour - vouschsafe to these, Parjanya, great protection. Those who have offered to the gods their bodies whose varied forms are all well known to Soma" [The Rig Veda (RV), translated by Ralph H. Griffith, New York, 1992, p. 647]. In the Rig Veda (RV: VIII.43.11) Agni is described as "fed on ox and cow" suggesting that cattle were sacrificed and roasted in fire.
Rigveda (10/85/13) declares, “On the occasion of a girl’s marriage oxen and cows are slaughtered”, and Rigveda (6/17/1) states that “Indra used to eat the meat of cow, calf, horse and buffalo.”
Quoting from Rigveda, historian H. H Wilson writes, “the sacrifice and consumption of horse and cow appears to have been common in the early periods of the Aryan culture.”