Lemme preface this by saying that I am a nerd.
Now, I would not be particularly put out if someone adressed me thusly. I’m proud of my nerdishness, and I certainly am on average nerdier than the average American. So, why would I get upset?
Take the racial epithets, f’rinstance. What they mean, literally, is, “person who is black/asian/Jewish/etc.” I wouldn’t compare being on the bad (well, worse, actually.) end of hate speech to being called a nerd, but is the principle of the thing totally different? Or, is being called a racial epithet offenceive because of the emotional baggage attached to the word, with no actual thought to the meaning of the word?
Well, it really depends on who’s using the language. For instance, if one of my friends called me a nerd in a certain tone of voice I’d know it was meant good naturedly and not take offence. If one black man calls another nigger it has a different connotation than if it was used by a man in a white robe and hood. Context.
If you feel free to use a term when talking to one of your friends, knowing he/she won’t take offence and will recognize it as a friendly anti-insult, no problem. But, if you start throwing words around with people you don’t know you may find somebody has been insulted.
I forgot to mention, some of those words don’t just mean "person who is black/asian/etc. They mean (at least when used as epithets) "inferior person who is black/asian/etc. That’s an important difference.
yeah, like photo said,
they mean more than their dictionary meaning
it’s the emotional baggage thing like you said, people can’t get past that.
the principle isn’t totally different, it’s that there’s more to it.
louis didn’t mind being called a nerd by booger or wermser, but he did mind being called one by ogre.
it’s an in-group out-group thing,
the in-group teasing of other in-group members is a means of fostering connectedness, while the teasing of out-group people is a means of fostering animosity, jealousy, and in-group cohesiveness.
political correctness assumes that we are all out-group to one another and thus must have our best manners present.
Yeah, what they said. 
Beyond that, I’d like to add that intent can even turn perfectly normal terms into insults. For example, if I explained to you very carefully that when I called someone a “beautiful and kind individual”, I meant that they were stupid and hairy and smelly, then I proceeded to call you a beautiful and kind individual, would you take offense? Words are just mechanisms for conveying ideas. It’s not the word that is offensive, it’s what that word carries that’s important. Denotation versus connotation.
Now, there are exceptions. Probably the most obvious example is the use of “nigger” amongst black people versus the same word used by white people. Some blacks will insist that white people do not have the right to use the word, because they don’t have the background of blacks and don’t understand and blah blah blah it’s a black thing, you wouldn’t get it. With them, it’s not about intent, it’s about empowerment. Disallowing whites from using the term is a way of gaining power over what they still believe to be the White Establishment. Personally, I don’t get it, but hey - it’s a black thang. 
Jeff
Jeff
robert, there is what amounts to an answer to your OP written in my first post to the other PC thread. (How many of these threads do we need, btw?)
Jeffe, the idea isn’t that “white people do not have the right to use the word,” the idea is that their meaning is very likely to be taken differently. Is that so very hard to understand?