Of all the assorted religious dogma I have encountered in my life, karma has always been the most appealing. If only it were true that one would be rewarded for good deeds and punished for bad ones. If only we had future lives to look forward to such that our status in each would be sort of a divine accounting of what we had contributed in our current lives. I find that notion so much more appealing than any of the Christian garbage about salvation through Jesus. Sadly, finding it more appealing doesn't make it true.
With the idea of karma, there is a certain inevitability of justice. If one screws up enough in life, there is no forgiveness and no absolution of "sin" gained by repeated hail Mary's or whatever other magic rituals one's priest commands. No deathbed confessions will save your ass. Your fate will be determined by your own behavior, just as it should be. The various Christian denominations seem determined to offer short-cuts - ways to get away with sin.
In a karma-based system, there are no short-cuts. However, there are plenty of second chances. One has an eternity to get it right, but one must change one's behavior in order to do so. No amount of belief is going to cut it unless there is real behavior change.
It is no surprise that Christianity is so big in the U.S. We're all about short-cuts, get-out-of-jail-free cards, and escaping responsibility for our actions. The Eastern religions of which karma plays a part could never endure here. They violate too many parts of our culture.
There are many problems with Christian morality, but the idea that one can escape bad behavior through belief seems to be one of those we do not hear enough about.