In the US, dairy products are pasteurized (or ultra pasteurized) to kill off bacteria - which also kills off enzymes, vitamins, and the natural antimicrobial properties that are the real benefit of drinking milk in the first place.
Dairy cows, like most farm animals, are fed large amounts of anti-biotics in their grain (called medicated feed.) Anything they eat, ends up in the milk fat. This includes the pesticides used to grow the grain and added hormones for milk production.
On many of the lists of foods that should be purchased organic, milk is present due to pesticide residue, herbicides, and dioxins.
Then we get to the old calcium argument - is the calcium in milk actually digestible? Results aren't conclusive, and you have to consider the source. A lot of dairy research released to the public is funded by the dairy industry.
A Harvard article states that dairy is not conclusively shown to reduce the risk of osteoporosis.
Calcium and Milk
On the animal rights side of things, the treatment of dairy cows is not particularly humane. Most dairy cows are pumped full of hormones to promote milk production, kept in tiny stalls, and bred each year to keep a constant supply of milk going. The central valley in California is filled with dairys, and many are near my house. They live in pretty filthy conditions and there are only a few free range dairys that I have ever seen. Most of the cows live in hutches, and the calves are usually taken away within the first few weeks of birth.
I have heard of pus and manure making it into store milk, and after seeing how closely cows are kept to one another, I would believe that claim to some extent.
I do drink milk, but on a very limited basis due to economic sustainability and the fact that milk really isn't very good for me or the animals it comes from. I encourage consumers to rely less on the arguments spoon fed to them on billboards and actually research dairy, especially the differences in dairy that has been pasteurized and homogenized, where vitamin D-3 supplements come from, etc.