Heaven and Hell of both Islamic and Christian are meritocracies, but the merit that qualifies people to get into one or the other is vastly different. Christianity and Judaism require belief and repentance for sins to get into Heaven. The merit is in the belief, not the actions. Islam goes a step further by requiring both belief and good deeds to outweigh bad actions. The merit is in the actions of the believer.
The Virgin Mary was impregnated by a supernatural being. This is well in line with Greek gods creating their demi-god children. Many pagan religions were inclusive and because Jesus was another demi-god to add to the party, he was accepted relatively easily. Judaism, on the other hand, was exclusive and a new element was not as easily incorporated, especially considering how much of the core mythology of Jewish culture involves God punishing people for not unquestioningly worshiping.
In Gilgamesh and the Illiad, there were intermediate states between gods and humans (eg Gilgamesh and Achilles) and the space between full gods and full mortals was less extreme. Most gods weren’t omnipotent and none had full power over the workings of the world. They and demi-gods were involved in human life on a personal scale; they got mad at humans, had quarrels with each other over human affairs, and fell in love with and cared for human life. The Judeo-Christian god, on the other hand, is a relationship represented in a simultaneously more internal and distant way. Judeo-Christian god does not interfere as directly, he sends signals and causes catastrophes. His fights with other supernatural beings don’t usually get humans caught in the crossfire – mostly because there aren’t too many other supernatural beings to bicker with. The gods in Gilgamesh and Illiad don’t care about the beliefs or motivations of humans, whereas Judeo-Christian god cares almost exclusively about belief and motivations of individual humans.