A bit of linguistic
foolishness that manages the miracle of annoying both Christians
in equal measure.*
For starts, we already have the word "good". It's a fine word, and doesn't need replacing with a term that means something entirely different.
Secondly, Christians feel that such usage is arrogant. It's not our place to judge whether someone is truly Christlike or not. (See Luke 18:18-25, where Christ reprimands the rich man by saying "No one is good but God alone.")
Thirdly, it saps all meaning from the word. If I say that I am a Christian, I'm telling you something about myself. You have an idea of what some of my basic beliefs are, and how I am likely to respond to certain situations. If "Christian" merely means "good", at most all I'm telling you is that in my opinion I'm a swell guy.
*The difference is that atheists see it as arrogance emanating from those damn Christians, whereas Christians see it coming from those who wish to subtly undermine their religion by replacing it with vaguely-defined well-wishing (much in the same way Christmas was transformed into a secular holiday based on consumerism).