The argument from nature (this one I hear from Dad over and over again)
Humans evolved as hunter-gatherers. Our bodies are adapted to an omnivorous diet, look, we have canines. It is natural for us to eat meat.
Yes we have evolved to eat meat. However, biology is not destiny. If you have ever used a form of contraception to allow you to have sex without the risk of bringing a child into the world, this shouldn't be news to you.
The assumption that what can be found in nature is good and right is common but misplaced. The naturalness of something should not determine if it is ethical. It is natural for the strong to take from the weak, for the elderly and those with disabilities to be left to die, for men to take women by force if they can't get them by other means. Unless you are prepared to argue that natural = good in all cases (in which case you are a psychopath and I don't want to talk to you), don't use it at all.
As for the canines, we also have an appendix. It is not unusual to be left with an evolutionary relic that is no longer needed, and certainly our canines are no longer needed; even for meat eaters, ripping and tearing uncooked flesh is something of an oddity.
The argument from religion - Genesis 9:3 - Every moving thing that lives shall be food for you. And as I gave you the green plants, I give you everything.
First of all, I consider the Bible to be a authored by man rather than God, so I reject the premise of this argument. However, if you are inclined to take it at face value, consider the following. The Bible endorses or condones many things that most people today consider repellent. To list just a small sample, they include slavery, genocide, the marring a rape victim to her rapist, and the stoning of disobedient sons. Many Christians are vegetarian, and find Genesis 9:3 or other passages insufficient justification for the eating of meat
The argument from nutrition - It's not possible/very difficult to get sufficient nutrients without eating meat. You can't get enough protein. Vegetarians always look sickly.
There have been a number of vegetarian/vegan Olympic gold medallists, some Australians include Lauren Burns (vegetarian, Taekwondo, 2000) and Murray Rose (vegan, swimming 1956 and 1960). Perhaps they were better off for their diets, perhaps they would have been even stronger with a bit of meat. It's impossible to know for sure. However, they managed to reach the pinnacle of human athletic achievement without meat consumption, so one could hardly call them "sickly".
Yes, it is true that some vegetarians have poor nutrition. Plenty of people have poor eating habits, vegetarians and omnivores alike. However, when an omnivore has poor nutrition, but this is not blamed on the omnivorous diet. There are plenty of studies that show that vegetarians are actually healthier than the average population. I'm not going to give them a huge amount of weight, since they are mostly observational rather than experimental: vegetarians may be more healthy on average because many people become vegetarian for health reasons, and are therefore more conscious of what they eat than the average person. However, to use some blanket statement of "vegetarians aren't healthy" or "vegetarians always look underfed" or "vegetarians can't possibly get enough nutrition" is just being ignorant.
The argument from food production - Without meat, we wouldn't be able to feed the planet's growing population.
This argument fails on basic chemistry. It may be valid if one day we can grow meat in a laboratory (which I wouldn't object to), but as it stands, meat is an inefficient way of delivering protein to the body. Sure, it delivers a lot of protein in one small packet, but how much protein (wheat, soy etc) did that beast have to eat to mature? How many acres of grass did that beast need to graze on, and could that land have been used for human edible crops? In his book The Ethics of What We Eat (2006, p. 232), Peter Singer explains how it takes on average about 13kg of grain fed to a cow to produce 1kg of beef, 6:1 for pork, and about 3:1 for chickens. Even if you are inclined to doubt these figures (though these are figures generous to the meat industry), it is clearly obvious that an animal will require more food than it produces. There may be exceptions where the raising of animals for food could allow more food to be produced than otherwise (e.g. goats/cattle on hillsides too steep for cultivation), but certainly, if we are serious about feeding the world's growing population while limiting the clearing of land, less rather than more meat is the answer.
The argument from enjoyment - Meat tastes good, so there!
I happen to agree that meat tastes good, but is that has ceased to be sufficient justification for me. Let me illustrate. Would it feel good to have personal slaves looking after your every whim? If their enslavement didn't bother you, as it didn't bother people in many cultures throughout history, then yes, it would feel good. Does this make it ethically good, or at least ethically neutral? Something that feels good can only be justifiable if it either causes no harm to others, or if the harm to others can be dismissed as unimportant (only a woman, only a slave, only a nigger, only an animal). Masturbation causes no harm to others = ok. Factory farming causes incredible levels of animal suffering = not ok.
If you think I've missed something important, feel free to let me know in the comments.