Quote
Now the man had relations with his wife Eve, and she conceived and gave birth to Cain, and she said, "I have gotten a manchild with the help of the LORD."
Again, she gave birth to his brother Abel. And Abel was a keeper of flocks, but Cain was a tiller of the ground.
So it came about in the course of time that Cain brought an offering to the LORD of the fruit of the ground.
Abel, on his part also brought of the firstlings of his flock and of their fat portions. And the LORD had regard for Abel and for his offering; but for Cain and for his offering He had no regard. So Cain became very angry and his countenance fell. Then the LORD said to Cain, "Why are you angry? And why has your countenance fallen? "If you do well, will not your countenance be lifted up? And if you do not do well, sin is crouching at the door; and its desire is for you, but you must master it." Genesis 4:1-8 NASB

At first looking at this passage it would appear that the difference is between the sacrifices. It could be that Cain brought just some of his produce not the "firstling" of it. This could mean that it wasn't of the best quality. It could also mean that at this time God had instituted animal sacrifice as the only approved sacrifice. This of course was changed during the covenant at Sinai. However, I think that Cain's ultimate undoing was related to his lack of belief. In Hebrews Abel is referred as righteous (Heb. 11:4) so he offered his sacrifice believing what God had promised. Cain didn't.

Peter

If you believe what you like in the gospels, and reject what you don't like, it is not the gospel you believe, but yourself. Augustine of Hippo