Well, this actually took off better than I expected.
Now, I let me say right away that some people totally missed the point. I am not here to ask why insults are bad in general (they aren't good or bad, they are downright unavoidable! ))), nor am I here to confirm that some people think that word retard is now being shunned because of political correct lingo. That's nice comment, and I am happy to re-affirm that, but that doesn't give me much to work with.
What I liked is comparing "retard" with "faggot" and "nigger". Those 3 words are connected in a way that they all work with bias, hate and shaming of certain groups of people. However, thinking about it further, I think that those 3, while being similar, are very different.
Take word "nigger". You would expect one to use it exactly on them black people to show your hateful disposition towards them, right? I mean, you won't come up to some white dude and call him nigger aiming to insult the fucker. he will just think you are crazy or extremely colorblind. So, this is out there with wetback, coon, gook and such. And you are expected to identify your target's racial background to pick a proper word.
Word "faggot" is, however, more universal. First of all, you would use them to offend them fuckers who bore you day after day with talks about how they are a sexual minority. I mean, that's exactly what the word means, in offensive way. However, in a group of straight guys trying to show off their manliness, this word would be thrown around to demean the other guy and tell that he's not exactly gay, but not enough of a man too. As Carlin once put it: "a fag was someone who wouldn't go downtown and help beat up queers." So you can use it for any type of guy if you feel that conditions are right enough.
But the word "retard"... I don't know about you, but I feel that we are never using it to insult people who are actually retarded. I mean, what would be the point? You know he is retarded, he knows he is... and if he doesn't, then he probably got severe retardation, can't understand you anyways and you are really the one who is a retard even trying to insult the guy like that.
But yes, from the common neutral medical term, word got negative context because of how people started using it. And I think... I think, the impact of the word (not counting the current intolerance of it) is in that you say to a person (and you know that he is not retarded, not in a medical sense) that he has mental disability, which is something you can't really fix, not with learning, not with medicine. I imagine, this is why it was used as an insult in the first place and why it works.
So, to sum it up, nigger you would use for blacks only, faggot you can use for any guy out there, and retard you would use only for non-retarded people.
However, I myself am straying from my path here. So, back to why it's so negative. I found this from wikipedia, and this seems like an interesting draft on the matter:
“Retarded” is the new “Gay” – Rough Draft
That further got me thinking. Word retard was used as neutral medical term in last century, it was then picked up and used as insult, and now it got so negative that political correct people are trying hard to scorn the word and people who use it. However, even if the word is gone, who can be sure that the same thing won't happen again?
Kip mentioned "Intellectual disability (ID), also called intellectual development disorder (IDD)"... (this actually reminds me of iddqd. Good times) this uses word "disorder", which is something I tend to frown upon, as I don't really believe in that wave of disorders people seem to push for these years... but that's different topic.
Let's assume, we got rid of "retard" completely. What stops people, in 10 years when term IDD becomes commonplace, from saying to their perfectly normal opponent "What, you got IDD or something?" And then it will go downhill, with term getting more and more negative. It's right there in the root of the term. Because, here's the thing. While there are whole movements dedicated to tell us that black people should have equal rights and there's nothing wrong about being gay, I do not think you can really show having a mental disorder in a positive way.
Personally, I feel, people, when looking for insults, just naturally drawn to need of offending their opponent's intelligence. I mean, isn't that the point? To show that your opponent is stupid and hence his argument is invalid? So I believe people will turn to the terms meaning lack of intelligence. To whichever terms are available and popularized.
Well, I kinda went all over the place there... But anyways, those are my current thoughts on the matter, I guess. I am unsure myself what to conclude out of my own theory, so, I don't know, feel free to comment on that, if you want, or not.