This is an enormous and wonderful subject!

In every system of religion, it is accepted that God is the Supreme Creator of all living entities. Any religion that does not accept the Supreme Creator as Absolute is a cheating religion. Only an atheist does not accept the Omnipotent Supreme Creator.

We are many, and God is one. So if we accept one God, then where is the chance of different religious system? People claim to follow a certain type of religion, saying formally, "I am Hindu," "I am Muslim," "I am Christian," "I am this or that," but almost no one follows the principles enunciated in religious scriptures. This is the disease of this age.
Here are a few fundamental lines of demarcation:

1. Personalist or Impersonalist

All religions are either personalist or impersonalist – they either believe that God is a unique personality, or they believe that God is not a unique personality. The latter believe God/Goddess is 'energy'… light, sound, universal form, "the universe", consciousness, etc. They don't accept that God is an independent personality.

In general, Christians, Hindus/Vaisnavas, Jews, Sikhs, and Muslims are all personalists. Buddhism and Zen are atheistic (no God).

Most of the earth religions and many of the esotericists (e.g. Theosophists) are impersonalists. Some call God "she" and assign a specific personality to the Goddess, considering her one with, or essentially the same as, Nature. They simultaneously recognize various personalities as also being Goddesses, but don't clearly differentiate between these personalities (i.e., which Goddess is THE Goddess, and by virtue of what qualities/characteristics?). Indigenous native traditions are also generally impersonalist, again because they consider the Great Spirit to be a universal energy embodied in nature.

Amongst all the personalists, only the Hindus/Vaisnavas have sastra (scripture) that very explicitly describes who God is as a unique personality: what He likes/dislikes, what his Form, Opulences, Pastimes, Paraphernalia and Associates are, etc. While some may believe that God is one and God is great, they can't explain just HOW God is great… other than to describe God in somewhat ambiguous terms like omniscient, omnipresent, etc.

2. Karma or no karma

Essentially all Hindus and Sikhs believe in reincarnation. When it comes to Muslims, Jews and Christians, some do and some don't. Many believe that scriptures were "adjusted" to eliminate reincarnation so as to give greater power to the living spiritual leaders. Buddhist also believe in reincarnation.

3. Serve or Be Served

One of the 'functional' divisions between religions has to do with the service/seva mentality. Are you the servant of God, or is God the servant of you? Do you offer your daily bread to God, or ask God to give you your daily bread? Do you pray to God to help you, save you, keep you healthy, make you happy? Or do you pray to God by only glorifying Him, never asking for anything and only offering to Him?

4. Time, place and circumstance

Religions also differentiate themselves in terms of when they believe creation started… in measurable or immeasurable time. Most Christians are locked into the 6,000 year garden of eden creationist approach, while Hindus/Vaisnavas understand time in cycles that are immeasurable, and millennially old. "Place and circumstance" refers to where the creation takes place – only in the universe we can perceive around us? Only on planet Earth? Or in a vast creation of innumerable universes beyond understanding.

5. Sastra (Scripture)

What books of authority a religion accepts, who they believe authored those books, and how they believe the Absolute Truth was passed down as pure, unadulterated knowledge has a great deal to do with how religions actually function, and how they compare to one another.

6. God's Pure Representatives

Another major differentiator is what religions believe about who/how/when God has sent His pure representatives to save humanity by mercifully bringing them the Absolute Truth. Christians believe that Jesus is the only Son, which fits nicely with their non-belief (at least scripturally) in reincarnation. Most other religions believe that God can't be limited in such a way… you can't tell God that He can only have one son. Rather, most religions believe that God sends his pure sons/representatives over and over, as needed, to re-establish the Absolute Truth when it has been lost to humanity. And sometimes God comes Himself rather than sending a representative. These major, personal manifestations are what we know of as God as the head of the major religions: Krsna, Allah, Jehovah, Buddha.