Nice is nice. I think people can be nice...... no doubt a whole lot nicer than we are. And while the Christian may have an advantage here, frankly I don't see that that reveals itself particularly (some of the 'nicest' people I know are not Christians). I believe that nice is nice - and should be strongly encourage - especially for the one we see in the mirror.
On the other hand, "nice" is not righteous. "Nice" is a totally outward thing as it can be observed. It has to do with our relation to others (Luther called this "justification with MAN"). Sadly, it CAN actually hinder our relationship to God (I'm not say it necessary DOES only that it CAN) as people become as the Pharisees - good looking on the outside, rotten on the inside - filled with egoism, self-sufficieny. I think of Jesus' parable of the Pharisee and Tax Collector where the Pharisee was the outwardly "holy" man who ends up in HELL, not justified with GOD... and the horrible Tax Collector ends up in HEAVEN, justified with God.
Let's say I have cancer. But I have no outward symptoms of such, no way that anyone noticed that. Outwardly, I APPEAR as healthy as can be. That actually can be a very dangerous thing - I can end up dying of something that might have been curable had it been obvious. Perhaps better is the one whose cancer immediately is apparent to all - so that he goes to the Great Physician? Remember.... it was the sinners, the tax collectors, the prostitutes who came to Jesus while the Pharisees opposed Him. Again, I wouldn't cast this in any absolute or "always" sense.
- Josiah