What if the fruit of his labor wasn't cabbages or pears? What if the fruit of his labor was his striving and wishing to be good enough to appease the angry God— for God to say, "With you I'm satisfied." What if God's disregard for Cain's sacrifice was God saying, "I can't delight in any work of your hands"? And Cain felt it as I do every day, the pressing in of inadequacy, the wondering if my sacrifices are good enough, and the fear that they are not. And what if the lamb of Abel's flock wasn't the right choice of profession or a sign of obedience? What if the lamb of his flock was an admittance that no fruit of his labors could appease the displeased God. Nothing but a lamb's blood, the lamb's blood, would lift God's face towards him. And I, with that blood spread across my doorposts, am then with Abel a delight to him.
I agree with Dennis, I have been troubled by God's rejection of Cains gift. Its his attitude rather than actual gift.
I have always been perplexed by this. A note in the New English Translation supports the direction of your thoughts on this. The LORDS’s problem with Cain’s offering had to do with faith and attitude.