I submit to you the key difference in this story was each man's standard of self-assessment. Everyone of us has a perception of our own character. We must be honest in our assessment of ourselves if we hope to grow into the kind of people God would have us be. The fatal error of the Pharisee was in making an irrelevant comparison of his character to that of other people. In making this comparison, he failed to compare himself to the true measure of righteousness, the word of God. Had he been holding his nature up next to the nature of God, he would have been as humbled as the tax collector and known how badly he needed God's grace. Then the goodness of God might have led him to repentance as is it's design (Rom. 2:4).
I don't know about you, but I sure am thankful I am not like that Pharisee. See what I did there? We too must be careful to assess our own faithfulness according to the proper standard. So often when we are confronted with our sins and weaknesses, we want to turn and say, "Well at least I'm not as bad as _________." Such a statement will carry no weight at all when we stand in the judgment. If you do not believe me, ask the Pharisee.
Let us remember that "...we must all stand before the judgment seat of Christ, that each one may receive the things done in the body, according to what he has done, whether good or bad" (2 Cor. 5:10). You will notice Paul didn't mention any relationship between the things others do and the judgment we each will receive. This does not mean we have no responsibility to correct one another. What it does mean is we need to be very careful to clean up our own back yard before we go knocking on our neighbor's door.
If we hope to be justified in the sight of God, we must examine ourselves carefully (2 Cor. 13:5). The Lord has provided us a spiritual mirror to help us see ourselves in an honest light (Jas. 1:22-25). Thankfully, He has made known the exact standard by which all men this side of the cross will be judged. As Jesus said, "...the word that I have spoken will judge him in the last day" (Jn. 12:48). Let us be sure we compare our faithfulness to His word, not the faithfulness of others. This is certainly what Paul meant when he asked, "Who are you to judge another's servant? To his own master he stands or falls..." (Rom. 14:4).