I enjoy reading about different theories of morality. Not, mind you, that the different theories usually come up with different versions of morality itself. Rather, they are different ways to think about morality, which in most cases match our moral intuitions and thus come to the same answers. The places where they differ are controversial questions, and that's part of the reason it's helpful to consider them methodically -- so that we have a way to resolve difficult questions besides just following our hunches.
One of the main divisions in ethical theory is between deontology and consequentialism. Deontology is morality that comes from law. That is, there are certain rules which may never be disobeyed. Consequentialism says that morality comes from the practical results of an action. Actions are judged based on whether they are likely to produce good or bad effects, and any action which is harmful is per se an immoral action. As an example, a deontologist will say, "Do not steal, because stealing is wrong," while a consequentialist will say, "Do not steal, because that will harm the person you steal from." (For more on consequentialism, this is an excellent short resource.)
Catholicism is mostly deontological -- there are laws which cannot be transgressed even for the most pressing reasons. The morality of an action is said to exist in the nature of the act itself, not in the possible results. The positive side of this is that it allows perfect moral clarity in almost all cases. Instead of having to ask, "Is it more important for me to save my own life, or the life of my unborn child?" a deontologist can simply know that anything which directly takes a life is wrong, while something that does so more indirectly (tube removal in the case of ectopic pregnancy, taking chemotherapy to cure the mother's cancer) may be permissible.
But even Catholics use consequentialism in many of their arguments. That's probably for two reasons: first, we all feel on an intuitive level that the results of our actions matter; and second, many people today are consequentialists and the intended audience for these arguments. Paul VI used both deontological and consequentialist arguments in Humanae vitae. His deontological arguments included references to Scripture, Tradition, and a philosophical understanding of the ends of the marital act, while his consequentialist arguments included predictions that birth control would weaken marriages, cause a disrespect for women, and encourage governments to mandate it for their citizens. When I hear arguments about birth control, they tend to focus on these latter arguments rather than the former, because people find them so compelling.
And it can be argued that there will be a consequentialist argument for every moral law of the Church -- because, after all, the reason God made these rules is because they will have good results.
Consequentialism places the moral value of an act in its consequences. This is practical and coherent with a theory that morality is simply those behaviors which work out well for the common good. The disadvantage, though, is that you can never know for sure what the consequences of your actions might be. When a deontologist votes for Bush because he is pro-life, and he later goes on to do little about abortion and also invade Iraq, he can say, "Well, I did my best, I couldn't have known, I obeyed the moral law, so I am blameless." A consequentialist who makes the same choice is faced with the understanding that he did something wrong, which, though not malicious, had bad results which he is responsible for. This present remorse may be a good motivator toward better evaluation in the future, but there is no guarantee of ever evaluating perfectly.
It seems to me that one of the biggest sources of misunderstanding between religious and secular people is a failure to recognize this deep difference in moral systems. For instance, some time ago I read a secular argument that Catholics don't really care about embryos because if they did they would spend more effort on preventing miscarriages than ending abortion, since more embryos die of miscarriage. Apart from practical considerations, this displays an ignorance of what Catholics actually understand their responsibility toward embryos to be. They don't feel like they are supposed to work toward a maximum number of babies making it to term (though that would be nice), but that they are morally forbidden from taking lives.
Likewise, on the question of socialism, a consequentialist may think, "It will benefit society if we tax the rich to build safety nets for the poor, therefore it is a good thing to do." But deontologists reply, "Taking from some to give to others is stealing, and stealing is bad." That's the end of it, from their perspective, unless their moral law is more complicated and has special rules for what the government may do. It frustrates me to see people making these arguments at each other while failing to realize the massive rift between their first principles. Perhaps an argument could be made, if the consequentialist in the discussion asks why stealing is bad, and if the bad effects of stealing actually hold true in the case of taxes. Or the deontologist might focus on the bad results of socialism rather than assuming that simply applying the word "stealing" ends the discussion. But unless this gap is bridged, all conversation on the topic is doomed from the outset.
I think a finely-honed deontological system, like Catholic moral teaching, is useful. But more simplistic deontologies are terrible. Why would anyone expect that a single sentence or paragraph would be able to summarize all moral actions? The non-aggression principle bugs me for that reason. "One must not initiate aggression against another's person or property" sounds great on paper, but it just leads to more questions. What is property? How do you prove that it's yours? Once someone else has aggressed against you, does your response have to be proportionate in any way, or can you shoot them for walking across your lawn? Do you have any positive duties -- for instance, if you can save another person's life at no cost or inconvenience to yourself, are you morally obligated to do so? And why is this the rule anyway? Was it handed down from heaven, or were its inventors seeking to achieve a certain goal with it? If the goal of the rule is "the benefit of everyone," then it's simply consequentialism, one step removed. But perhaps the real goal is "a moral justification for libertarianism, the political system which I happen to prefer," in which case calling it morality is a bit of a stretch. Any simple deontologist dictate tends to devolve in this way. Morality is complex; rules must also be complex. But in order to shape the rules to the sorts of choices we face, we must have some sort of higher goal to which we subject the rules we make.
On the other side of the question, consequentialism has serious flaws as well. In focusing on the results, it's easy to ignore the human actor. For instance, it fails to distinguish between positive and negative morality. It seems obvious to me that there is a huge difference between killing two innocent people in order to save ten people, and saving ten people while accepting the inevitable consequence of two people dying that you couldn't save. That's intuitive to me, and I think it comes from a recognition that killing two people has an effect on me that not saving those people won't have. It changes me into the sort of person who kills people. Because that's a consequence, of course consequentialism can account for it, but most consequentialists don't, and that's a problem.
You see, there must be a difference between positive and negative morality, or else the five dollars I spend on a tub of ice cream which I could have spent on a measles vaccine for a kid in Africa is morally equivalent to murdering that kid for a tub of ice cream. I can't get behind that. Never killing anyone is a law I can reasonably be expected to achieve, while saving everyone is not possible for anyone. Heck, if saving a greater number of people is all that's required, then I should donate my body for organ transplants, thus saving several people for one life lost -- one for my heart, one for my liver, and so on. It devolves into absurdity.
Another issue is that I am not always in a state where I can reason about consequences. Some moral choices are made in a heartbeat where I have no time to think; others in a situation where I don't have very much information; others when I am under the strong duress of my emotions and thus tempted to rationalize something I would otherwise agree was wrong. That's why rules are helpful -- if you have rules for your life you follow all the time, the force of habit will carry you through at those moments when you can't clearly assess the consequences.
That's why I have such a strong sympathy for rule utilitarianism -- consequentialism is used to shape the moral rules which one lives by, but after that one simply follows the rules. Your choices shape the person you are, and the person you have become shapes your future choices. That means that even small choices that "don't hurt anybody" may come to have serious consequences down the line, when your failure to be disciplined in little things has made you a weaker person.
But, in the end, this post doesn't have a hard and fast conclusion. Both deontological and consequentialist moral systems have flaws, but they both can be useful if they are influenced by one another -- when consequentialist morality acknowledges the usefulness of rules, and when deontological morality recognizes that many rules are only shown to be good by their good results.