Torture Bad. Interrogation Good.
Torture is one of those things that's too obvious to talk about it. Obviously, it's a bad idea. It's not a good thing to do, either morally or from a practical standpoint. It doesn't produce good information. It encourages the other side to torture your side, if they weren't already, and takes away your side's moral high ground, if they were. It's bad for military discipline. It tends to bleed into everything else those people do afterward. It is, as Bujold put it, "an infection of the imagination".
On the other hand, interrogation is good. It's just that you really shouldn't be doing anything rougher than the police are allowed to do. All the American experience with interrogation has shown that tea and sympathy work a lot better than torture at getting someone to talk, which your Aunt Gladys could've told you long before the FBI found it out.
The real art of interrogation is learning to climb into someone else's head. This requires a certain amount of empathy. It's uncomfortable to empathize with Really Bad People, and it's hard work. But it's better than not empathizing with them, and becoming a Really Bad Person yourself.
To be completely honest, I suspect that a lot of those people who advocate stuff like serious sleep deprivation, disorientation, and waterboarding don't really understand what they're advocating. The reason they don't understand it is that a good number of them have undergone certain of these techniques as part of the escape and evasion training. They escape, they evade, they get captured, they get "tortured", they sign their little paper and wish they'd been tougher, like that Ranger team that broke out. It's more like Hell Week than going through hell.
But even the most detailed simulation is just a simulation. Deep down, American soldiers in training know that unless there's a training accident, it will end happily in the near future, and then everything will be all right. This is just something they have to get through.
Deep down, they know that they themselves don't really mean to hurt the Bad Guy, either. But they don't have the Bad Guy's medical records, and they don't have as much experience as the trainers did at doing this stuff safely. (And let's be honest, they don't really have as much incentive not to really hurt the Bad Guy as the trainers had not to hurt them.)
Meanwhile, the Bad Guy has no reason to believe that things will ever be all right again. (If he really believed that Americans were the Good Guys, he wouldn't be fighting them, ne?) So for them, it really is torture, not just an unpleasant and scary experience which will end in the near future.
There's a big difference between your brother twisting your arm behind your back and a scary drunk guy with a gun twisting your arm behind your back. Even if the force being exerted is the same.
(Btw, that torture legislation of McCain's was incredibly bad law, as written. Sounds like it will encourage soldiers to prefer shooting people to capturing 'em.)
UPDATE: A lot of these ideas came together after reading this post at Blackfive. I have all the respect in the world for Blackfive, needless to say, but on "coercive interrogation techniques" we clearly disagree. I freely admit this may have as much to do with my low threshold for pain and high incidence of being beat up and taunted as a child as for any kind of respect for natural law, human dignity, and the teachings of the Catholic faith. :)
However, my younger brother wished to point out that it's equally dangerous to have people around who have no idea at all how much pain, damage and death certain simple actions can cause. I agree; the kids dying from choking themselves for fun are a perfect example.