Moxie, you are putting a lot of different things in the same bag.
The first confusion is to equate a result of hundred of years of social and language evolution to the complaint of one small given group of individuals. That is exactly what you are doing when you quote a lot of different words together. I am sorry, we haven´t "learned" that nigger is an offensive word. That word was meant to be offensive, people chose to use it (in the past and even nowadays) because of its offensive content. What society learned (at least a bit, luckily) is that different races are no inferior to each other. The language, and thus the choice of words, is a consequence of social evolution. American, African, Brazilian, black, white or yellow designate different human groups, and therefore have no negative content in themselves. If you use a pejorative word to refer to one of those groups, it means that you actually think or infer that this group is in someway inferior, and there lies the prejudice and/or the offense.
Unfortunately, this is not the case for the word "retard". I sympathize with the ones with mentally handicapped people in their families (I have friends in this condition), but there is a negative content in the term "mentally handicapped". This term is polite, it reminds the listener that we are talking about a human being. "Retard" is not polite, and refers only to the negative aspect of "mentally handicapped", which, unfortunately, is actually there. You cannot say the same for the terms which describe ethnic groups.
Second, when you say "minorities" you are referring to a lot of different groups. Those groups are so different (as the criteria to define them are different: sexual orientation, gender, race, physical/mental characteristics, political/religious orientation) that term itself has almost zero meaning.
Of course I agree with you that Tomic was rude, and maybe he would express himself differently if he had a brother in this condition (but maybe he wouldn´t). But I fall back to one of my first points, you cannot force people to be nice.
Nobody (well, I guess at least nobody here) is disagreeing with the content of the message (that is, Tomic was rude). What people don´t like is the tone, and the attitude. Nobody gets to chose other people´s words, period. If you look at your posts, you´ll see that first you said that
"there are plenty of perfectly good ways in English to describe what Tomic felt at that moment." and then "no one is going to be the more restricted by retiring the notion of calling someone, or himself, a "retard." ". Those two phrases have one thing in common: you assume that you know how the person feels ("retard" or not, restricted or not). That is what I call arrogant.
Moxie, I am only replying to you, and being so picky, because having read a lot of your posts, I know you have the best intentions and understands a lot of issues very well. A lot of people that would replicate your position here don´t. I do not like the fact that people offend each other, and I do not support them to do it. The issue here, unfortunately, is not that simple (I know you know that...).