Heaven isn't for "good" people, hell isn't for "bad" people. One theologian put it this way: "Who will be in heaven? The best answer is people who would enjoy being there." That may seem obvious, but it's really not so. Let me clarify (or confuse) by paraphrasing CS Lewis's take on this issue: In the end, there will be two kinds of people - those who said to God "Thy will be done," and those to whom God says "Thy will be done." That means this:
Those in either place will have chosen that destiny - nobody condemned to hell will be there against their will - they will have chosen it - not once, but many many times throughout their lives as the Holy Spirit tried repeatedly to convict the individual of his/her need of God. Nobody goes to heaven or hell against their will. So, back to the first anecdote I gave: not everybody would enjoy heaven - because heaven is a place where you are not the center of the universe, and some people couldn't stomach that - the old "It's better to reign in hell than serve in heaven" idea.
Heaven is the reward for accepting Christ's gift of salvation through his sacrifice at Calvary; hell is the chosen place for those who continuously reject God's invitations to avoid that fate by accepting Christ's grace and forgiveness.
Non-believers tend to make God out to be this vindicitve referee who "sentences" victims to hell and "mindless robots" to heaven. Far from it - both destinations are chosen by both parties.
Finally - it is not the absence of "big sins" (though all are equal in God's eyes - sorry, but you may find some converted Nazi concentration camp guards among those in heaven...) that keeps one from hell (just as it's not "big sins" that get you there) - it is a relationship with (or a refusal of that relationship) with God/Christ.

