The story is intriguing. I don't know why they originally incorporated the Heaven/Hell concept, unless it was to capture a more mainstream audience. The story has so many non-biblical leanings, I think they should have just done away with any of it. However, they surprisingly use little bits and pieces of actual historical writings and Scriptures.
The story to me is a lot like the Sumerian writings where the "angels," led by EN.LIL, had decided to wipe out mankind but one of them rebelled against them (EN.KI) and took our side. In D3, the Tyrael character plays the role of EN.KI while Imperius plays the role of EN.LIL. So it's a lot like the Sumerians wrote about, but with a biblical twist for enhancement and to maybe give it a more familiar feel.
I do like how in D3, super powerful humans were created called Nephalem which does match canonical and non-canonical writings from antiquity. But in D3 the Nephalem were created by the union of an angel and demon, while in non-canonized Scriptures (the Book of Enoch), the Nephalem were born when angels shacked up with female humans. And in the Book of Enoch, these Nephalem were demi-gods and grew to 45 feet tall (or 450 feet tall if you do not believe there was an error in translation... the translated book literally says 300 cubits or, 450 feet).
Every one who is seriously involved in the pursuit of science becomes convinced that a spirit is manifest in the laws of the Universe-a spirit vastly superior to that of man. - Albert Einstein