Karma is a slippery subject. Many Western Buddhists (a la Stephen Batchelor) have chosen to jettison this teaching entirely, while others prefer a watered-down, science-oriented interpretation. And then there are the hardcore believers, those who see the Dharma as an indivisible whole; to them, choosing which parts I "like" and which I don't is just a skeptic's form of cherry picking. Honestly, I don't know where I stand on the issue of karma. Every time I try to accept the teaching, I find myself feeling grossly superstitious, even naive. The idea doesn't sit well with my modern, Western sensibilities. I'm entirely too rational (and this isn't meant as a compliment) to believe that the circumstances of my life are determined by my previous actions. There are too many logical loopholes for me.
For instance, if I'm walking down the road and decide to punch some random person in the face, is that the other person's karma bearing fruit? Traditional Buddhist doctrine says yes. But if so, then why should I accumulate any negative karma, since it was the other person's karma "calling" me? And what does this say about freewill?
Another problem I have is, if karma explains why everything happens (at least the events beyond our control), then what about hurricane or tsunami victims? Did these people "deserve" their plight because of the actions in their previous lives? If so, isn't that a bit convenient (all those people with bad karma being in the same place at the same time), not to mention deterministic? I suppose that you could argue your way around that one: that karma is being calculated on some kind of cosmic abacus, and every time something "bad" happens to you, it cancels out a bad deed from you past. But come on.... Really? Isn't that a bit orchestrated?
And another puzzling dilemma is, if I'm constantly changing, then why is karma still "trailing" me, so to speak. Every seven years my body recycles its cells, so why not my karma, too? Shouldn't it have just be washed away or something? I'm sure there's some technical Abhidharmic explanation for all this. After all, greater minds than mine have pondered this, I'm sure. Another related rub is, if, on an Absolute level, I am the entire universe (as Mahayana tradition teaches us), then how does karma return to me? How does the universe know who I am? What, does it have some kind of karmic homing system?
I've heard Thich Nhat Hanh explain karma as simple cause and effect. For instance, you harbor ill will towards someone, so you feel angry--a painful emotion. The old, you reap what you sow proverb. Maybe he's watering it down for Westerners; I don't know.
I'm a little fuzzy on the whole rebirth thing too, but I can accept rebirth much more easily than I can karma. I don't know why, karma just makes my brain cramp. It feels like a relic of ancient Indian culture, political dogma used to justify the caste system. We do have to remember that the Buddha lived 2,500 years ago, in a time dominated by superstition and mythology. Hell, they didn't even know about germs back then! I'm not suggesting that the Buddha was ignorant, but he certainly wasn't omniscient: the Abhidharma is rife with scientific inaccuracies concerning the material elements, methods of procreation, etc. Today, in light of our scientific understanding, Western Buddhists easily overlook these pre-scientific explanations as being emblematic of the world the Buddha lived in. Kind of like cultural leftovers. So why is karma any different? (See Stephen Batchelor's Buddhism Without Beliefs and Confession of a Buddhist Atheist for more on this.) Just a consideration.
What I appreciate so much about Zen is how little emphasis it places on these doctrines. It's not like there's someone waving a stick over your head, making sure you're a good Buddhist Kind of believer. In Zen, you can take it or leave it; make as big or little of a deal out of it as you want. What matters is the practice.
Recently I heard a funny story about this subject. It was about a Buddhist British soccer coach who made the mistake of telling the press his feelings on karma. He said that people who are less fortunate than him deserved to be because of karma. To him, blind people are blind because they "earned" it. The same goes for poor people, victims of accidents, and so on. The funny thing is, the coach got fired for saying this. Talk about karma biting him in the ass!