1. "Are you drunk or stoned?"
Everybody has bad days or days when they are not feeling well and this is not a nice thing to say to anybody, even somebody who has a reputation for being this way. It is a harsh character judgement and implies you abuse substances to avoid coping with reality. Especially to a metalhead, this is a tremendous insult because it implies that you are a directionless degenerate just because of the music you listen to. Let me just say this, Kanye West is more likely to provide the soundtrack to a teenager's dropping out of high school than Black Sabbath.
2. "It's only metal. It doesn't mean anything."
Music is a source of inspiration and guidance to everybody to some degree. Just because somebody listens to metal doesn't mean that they aren't getting as many morals out of their music as somebody who listens to gospel music. It's all in your interpretation of things.
3. "This song was written a long time ago. It's content is obsolete."
Many challenges faced by humanity nowadays were the same ones faces by humans a long time ago. The song might be old but the issues addressed remain the same. Why is the speakers voice any less important just because it occurred some time ago?
4. "Ozzy Osbourne bit the head off a dove/bit the head off a bat/urinated on the Alamo/etc."
Freaking get over it. Did it hurt YOU in any way?
5. "This music is terribly satanic."
Nobody took satanism seriously. It was all a part of an image and of publicity stunts. Many metal musicians come from strictly religious and proper households and maintained that lifestyle in their free time.
"I don't wear a pentagram because I worship the devil. I wear a pentagram to piss people off." -Nikki Sixx
6. "Rock stars exploit women."
It isn't exploitation if it's voluntary. Don't judge the whole sex of females by the actions of these women, look at them as individuals whose actions are only affecting themselves.
7. "I don't understand the makeup/hair/costumes."
I don't understand Lady Gaga's meat dress or why all of One Direction dress the same way.
8. "Musicians make more money than they deserve."
A large percentage of musicians donate money to charity or even run their own organizations. Tons of them are spokespeople for important causes. Not to mention they work their asses off touring and making a name out of themselves and that they deserve any penny they get, and not to mention the fact that not all of them have as much money as you'd think they do.
9. "Pop music is better."
Playing good heavy metal requires skill, talent and effort. Pop music requires computer and some geek who knows how to use one. Any questions?
10. "You must not be all too smart because you listen to this kind of music."
I have no idea how metal fans became associated with stupidity because, I can assure you, the grand majority are not. Ones taste in music does not necessarily reflect their IQ.