Hi, let me clear up some misconceptions.
For a start, HINDUISM IS NOT POLYTHEISTIC.
Hindus believe in ONE SUPREME BEING. This Supreme Being reveals itself in different forms. These different forms are the various Gods and Goddesses. For example, one single beam of light is refracted through a prism. This ONE beam of light splits into seven colours. However, these aren't separate beams of light - they are still the one beam of light. Likewise, the One Supreme Lord reveals Himself in a multitude of forms. The Rig Veda states: "Ekam Sat Vipra Bahuda Vadanti" - Truth is one, the wise perceive it in different ways. Lord Krishna, an incarnation of God, states in the most widely read Hind)u Scripture, the Bhagavad-Gita: "Whatever form any devotee with faith wishes to worship, I make that faith of his steady. Endowed with the faith, he engages in the worship of that form, and from it he obtains his desires, which are being actually ordained by Me". (Bhagavad-Gita 7:21-22 There is no such concept of "multiple Gods" in Hinduism.
There are some Hindus who believe that Lord Krishna alone is the Supreme Lord, and that the other Gods such as Lord Shiva and Lord Ganesha are His servants. However, they still believe that Krishna is the One Supreme Being without a second and put the other Gods in the position that angels occupy in Christianty.. This is the position held by the "Hare Krishnas" (International Society for Krishna Consciousness or ISKCON).
Was it the religion that says; when you die you reborn and every single human is a reborn?
I wonder how do you explain population growth.
Reincarnation and population growth are easily reconciled in Hinduism. This world is only one of many according to the Hindu Scriptures. This material world is among the lowest of the worlds. Maybe souls are falling from the slightly higher worlds or mabye those souls in Naraka (hell) are being reborn here after being punished for what they did.
I'll also add that Hinduism references a lot of geographic locations specific to the Indus Valley area. The Ganges River, for example, doesn't have quite the same 'oomph' to me as it would someone who lives in India.
And the Bible references a lot of geographical locations specific to Israel. Jerusalem doesn't have quite the same "oomph" to Western Christians as it does to someone living in Israel. Mecca doesn't have quite the same "oomph" to Western Muslims as it does to someone living in Saudi Arabia. However these are still treated as holy sites by the followers of these religions. Ganges water is available in other countries.
There are Hindus all around the world who happily practice their religion. Nothing says you must live in India to practice Hinduism. (I, however, would like to move to India because it is the holy land of Hinduism, and also because there are more vegetarian restaurants and things like soap etc there don't contain animal fat.)
It seems that any godform that brings a human closer to Brahman (or Vishnu, or Shiva, whatever your choice high god might be) would be valid. If all of the gods of India are simply masks or facets of Brahman, then why wouldn't all godforms be facets of Brahman, even self-created ones?
Some Hindus do believe that - you can worship any form you want and still get the same results. I read about one man in India who keeps a picture of Elvis in his prayer room and observes his birthday and day of passing. I'm assuming that he offers puja to Elvis the same way he offers puja to the Gods.
Why couldn't the Tennessee River be a holy river?
Well, no-one has ever created a religion with the Tennessee River as a holy river!
The Ganges River is regarded as Goddess Ganga, who flows through the hair of Lord Shiva. The Ganges is regarded as a Goddess and that is why the river is so sacred. But Hinduism teaches that God is situated within the hearts of all living beings, and within every atom.
Hinduism seems both very ethnic and culture based, yet universal at the same time. Gives me a headache.
The teachings of Hinduism are universal. It is the oldest religion in the world. However, the Hindu religion and Hindu culture are inseparable. Indians know this. Indian Muslims in Malaysia have tried to be recognised as "Malays" because they know that Indian culture IS Hindu culture and they reject the Hindu religion. I've heard that Indian Muslims in India claim that they are descended from Arabs for the same reasons.
What turns me off is how distant and unapproachable it seems. There are so many names of devas, so many different cultural practices throughout India's history. It's such a diverse religion that it seems untouchable. I feel like I'd have to be born in India's culture to understand it with any depth. There are (were?) caste system issues. Nearly every time I talk to a Hindu, they go against everything I heard the last Hindu tell me about Hinduism, including the very core, most fundamental concepts like what the end goal is or what god really is.
I have had similar sentiments before. Especially here in Malaysia, Hindu pujaris may only speak Tamil. Most books about Hinduism are in Tamil (although there are quite a few English ones). However, it's not impossible for a Westerner to become a Hindu. You just need to know Hindus who will help you on the path. If possible, marry a Hindu who was brought up in a religious family. My wife is Hindu but she doesn't know mcuh about the religion as her family aren't very religious (except for her grandfather, who lives in another state).
Kauai's Hindu Monastery has many online books about Hinduism written in English, and they publish a quarterly magazine called Hinduism Today in English. However, I've noticed that there are many books about what Hindus believe, but almost none about how to actually practice the religion - how to set up an altar, how to perform puja, what all the prayer instruments you see in shops are for, how to celebrate Deepavali, Holi, Thaipusam and the other Hindu festivals, and other information that is usually passed on from parents to children (so obviously wouldn't be available to a convert). I'd like to write such a book myself (when I've found out all of the above!).