Okay, this thread is a bit heated, but I've heard a lot of interesting opinions. Peeps, stay civil, it's no fun otherwise.
The biggest disconnect between vegans and... uh, the rest of us ("Carnists?" Really?! Did we need an "us and them" category based on dietary habits?) is in trying to apply the morality one usually applies to his fellow homo sapiens to all living beings on the planet (plants excluded because... fuck plants?). This approach inherently demands that they call out normally moral and gentle people as "monsters, murderers, butchers, carnists," or what have you. In essence, vegans want everyone to undergo a paradigm shift.
The merits of this paradigm shift aren't unfounded, but they're still very shaky. The right to not be murdered, which modern civilization has graciously extended to every human being, is itself an unnatural concept, but very much necessary because humans aren't driven by "natural" desires (i.e. we kill each-other for reasons other than food). We already recognize the capacity of animals to feel pain, and we've implemented laws against animal cruelty (to which I don't think anybody here objects), but the extension of full human rights beyond the species barrier is unfounded on a planet where the very ecosystem is based on murder. I haven't seen a vegan condemn a lion for butchering a gazelle, even if we'd definitely condemn a man for killing and eating another man for survival. And if the gazelle is a creature worthy of human rights, don't you think it minds being killed, even for food? How is the lion defensible on those standards?
The other problem is that there would be no clear line after which we'd have to stop applying those rights; if it's not sapience and self-awareness that determines the right to live, what is? An intelligence exam? A nervous system? Ganglia at least? A metabolism? The ability to move? The entire endeavour is a can of worms.
What is a fair point is that the modern meat industry is based on excess. It's definitely not cruelty-free, it's destructive, polluting and hardly justifiable. A lot of our objections to slaughterhouses come from humanizing the animals, so I wouldn't call them rational objections, but even if we knew for sure that every animal except us is a thoughtless carbon-based automaton, keeping them in those conditions would be a hard sell. Justifying veganism with environmental and societal concerns is okay by me. The moral aspect, however, is a million times more complex than most vegans admit, and it delves deep into the definition of life and sentience, something that no one seems to be eager to get into over a dietary habit.