I'm afflicted with the disease of having to reason out everything that I do, and so I've been devoting some thought to vegetarianism.
Some (most?) of the Hindu vegetarians that I know do not eat meat simply because their religion prohibits it. That's it. No further reasoning is offered or needed. They have heard the word of God and they're going to follow it. Clearly I don't have much respect for this point of view — not eating meat because your religion prohibits it stands on no firmer logical ground than burning widows alive along with their dead husbands because your religion requires it (to choose a completely random and unbiased example :-)
Some Hindu vegetarians would go a little further to say that taking life is forbidden. But the same people would not think twice about swatting a mosquito. AFAIK, Hinduism considers all life to have an atman/soul, and in some sense interchangeable through reincarnation. Bingo, contradiction. There are some extremists who would not even swat a mosquito, but this position is utterly logically indefensible. If you fell ill you wouldn't be allowed to take medicines because you would be killing the billions of viruses in your body or whatever. The only way to avoid killing any anything at all would be to commit suicide. Oh wait...
Returning to sanity, let's make more reasonable arguments. First, not all animals are equal — species that are more intelligent/social/companionable are more valuable. So it is OK to squash a bug but not to kill a cow. I guess most people, vegetarian or not, would agree with the general principle, as evidenced by the outcry over dog meat, although they would differ on where to draw the line. Secondly, it is not the killing of the animal, but the cruelty of the industry, that is offensive (after all, all species that are killed for meat have predators in the wild as well).
Again, there are some who try to take this argument to its logical extreme. Cows raised for dairy suffer just as much as cows raised for meat. The same applies to hens raised for eggs. "I will not support any industry that practices cruelty to animals" is the stance of some vegan acquantances of mine. But then you'd have to not buy leather or fur, not eat at any restaurant that serves meat even if you don't order it yourself (because your eating there contributes to the restaurant's financial health and will lead to more animals killed in the long run), not own shares of any company related to the food industry, and so on... the only way to rigidly follow that principle is to not live civilized life; you'd have to move to a deserted island or a forest and eat fruit all your life.
There are some amusing contradictions that inherent to fundamentalist vegetarianism. Suppose you're on a plane where they don't carry vegetarian meals. I've seen people eat only the vegetarian items in their meal, not feel full, and then ask for another meal and proceed to do the same thing. Not only does this not satisfy the goal of not contributing to the killing of an animal, it will in the long run result in more animals being killed because the airline will stock up more meals due to the increased demand.
The only way to be a vegetarian and be logically consistent, then, is to acknowledge that your principle takes a back seat to convenience. (Notice that all along I have only been trying to reach a logically consistent position, one without self-contradictions, and not trying to figure out what is the "right" thing to do, which is clearly subjective). So its OK to consume dairy and eggs (a single cow can produce lots and lots of milk but only so much meat), it's OK to eat meat when the only vegetarian item on the menu is fucking fries, and its OK to eat meat when the animal is dead anyway and your abstaining does not make an iota of difference.
Meat eaters encounters far fewer logical dilemmas. There are some who assert that animals are property and any animal suffering is acceptable, but I presume they are few in number. "Any animal suffering is OK as long as it is not wanton" is probably a position with considerably more support. I suspect that a common (unstated) position is "any animal suffering is OK as long as I don't have to see it", reflecting a cowardice in facing ethical issues and/or an inability to use the left half of the brain.
On a side note, the principle that more intelligent animals are not OK to kill underlies what I consider one of the funniest moments in the Hitchhiker's Guide series. A cow that wants to be eaten is on the menu at the restaurant at the end of the universe. Zaphod argues that eating a cow that wants to be eaten cannot be more unethical than eating a cow that does not want to be eaten. Arthur finds the idea abhorrent, but cannot articulate why (mirroring the intended reaction of the reader). The reason for Arthur's queasiness, of course, is that the capability of wanting to be eaten implies a level of mental development that makes it ethically problematic to kill the creature. Eliciting an emotional response from the reader, forcing them to attempt to justify it and have them come up empty handed is the sort of brilliant humor that makes Douglas Adams unique.