The same moral arguments keep cropping up in multiple threads, and responding to them all separately seems inefficient. Here's an attempt to summarize my views and head off a lot of identical conversations:
As I said before, I'm operating off of Preference Utilitarianism. I am still in the process of working through that (it's only recently that I tacked on the word "preference," I'm not 100% sure it solves the problems I wanted it to solve). But I strongly believe that the happiness and suffering of others IS important.
That is not a fact I will try to convince someone I am right about, because I don't think it's something that can be proven. But it's not a leap of faith either - it's simply how I feel. Most humans seem to feel the same way. If you don't, that's fine. But saying "there's no reason to think that" is likewise irrelevant. It doesn't matter whether there's a reason to care. All it matters is that we do. If you honestly do not feel compelled to worry about the suffering of others for their own sake, I won't argue with you. But if you DO feel compelled to worry about it, and are avoiding it because "it's just an emotion," then I think you are missing the point. At some point you need to define your goals, and figure out where they conflict with each other, and it's perfectly okay if "not cause unnecessary suffering" ends up being one the things you arbitrarily care about.
I don't really want this to turn into a discussion about morality in general. If the subject comes up in the various subthreads, please try to limit the discussion to "I do/do-not care about suffering of others" and then argue about the consequences of THAT (i.e. if you care about suffering of others, but only other humans, or some other group, explain why you draw the distinction). If you don't care about it, then the animal rights side of the issue is largely irrelevant to you and I won't press you on it.
If you want to argue about the merits of caring about others' suffering in the first place (or in particular with the statements I just made) then try to keep it under this individual reply, so it stays fairly focused and doesn't gum up the rest of the conversation.
i.e. if you care about suffering of others, but only other humans, or some other group, explain why you draw the distinction.
I care less about the suffering of some groups, but can't really explain what criterion I use (and am in general wary of coming up with simple rules). I can explain why from an evolutionary point of view, it makes sense for me to care less about those who are only distantly related, and are unlikely to punish me if I'm not nice to them. I agree that this "why" is probably not the one you were asking about.
(Note: I wasn't quite sure whether this warranted a high level post or just a discussion. I haven't made a high level post yet, and wasn't entirely sure what the requirements are. For now I made it a discussion, but I'd like some feedback on that)
I've been somewhat surprised by the lack of many threads on Less Wrong dealing with vegetarianism, either for or against. Is there some near-universally accepted-but-unspoken philosophy here, or is it just not something people think of much? I was particularly taken aback by the Newtonmas invitation not even mentioning a vegetarian option. If a bunch of hyper-rationalists aren't even thinking about it, then either something is pretty wrong with my thinking or theirs.
I'm not going to go through all the arguments in detail here, but I'll list the basic ideas. If you've read "Diet for a Small Planet" or are otherwise aware of the specifics, and have counterarguments, feel free to object. If you haven't, I consider reading it (or something similar) a prerequisite for making a decision about whether you eat meat, just as reading the sequences is important to have meaningful discussion on this site.
The issues:
1. "It's cruel to animals." Factory farming is cruel on a massive scale, beyond what we find in nature. Even if animal suffering has only 1% the weight of a humans, there's enough multiplying going on that you can't just ignore it. I haven't precisely clarified my ethics in a way that avoids the Repugnant Conclusion (I've been vaguely describing myself as a "Preference Utilitarian" but I confess that I haven't fully explored the ramifications of it), but it seems to me that if you're not okay with breeding a subservient, less intelligent species of humans for slave labor and consumption, you shouldn't be okay with how we treat animals. I don't think intelligence gives humans any additional intrinsic value, and I don't think most humans use their intelligence to contribute to the universe on a scale meaningful enough to make a binary distinction between the instrumental value of the average human vs the average cow.
2. "It's bad for humans." The scale on which we eat meat is demonstrably unhealthy, wasteful and recent (arising in Western culture in the last hundred years). The way Westerners eat in general is unhealthy and meat is just a part of that, but it's a significant factor.
3. "It's bad for the environment (which is bad for both human and non-human animals)." Massive amounts of cows require massive amounts of grain, which require unsustainable agriculture which damages the soil. The cows themselves are a major pollution. (Edit: removed an attention grabbing fact that may or may not have been strictly true but I'm not currently prepared to defend)
Now, there are some legitimate counterarguments against strict vegetarianism. It's not necessary to be a pure vegetarian for health or environmental reasons. I do not object to free range farms that provide their animals with a decent life and painless death. I am fine with hunting. (In fact, until a super-AI somehow rewrites the rules of the ecosystem, hunting certain animals is necessary since humans have eliminated the natural predators). On top of all that, animal cruelty is only one of a million problems facing the world, factoring farming is only one of its causes, and dealing with it takes effort. You could be spending that effort dealing with one of the other 999,999 kinds of injustice that the world faces. And if that is your choice, after having given serious consideration to the issue, I understand.
I actually eat meat approximately once a month, for each of the above reasons. Western Society makes it difficult to live perfectly, and once-a-month turns out to be approximately how often I fail to live up to my ideals. My end goal for food consumption is to derive my meat, eggs and dairy products from ethical sources, after which I'll consider it "good enough" (i.e. diminishing returns of effort vs improving-the-world) and move on to another area of self improvement.