Why? Asking as a non-native English speaker, please excuse me if it's obvious. Is "retarded" somehow worse than "fucked up" or simply "shit", which seem to be accepted in casual conversations around here?

Yes. It is considered by the mentally disabled community to be similar to the n word. It is a word that was used to marginalize an entire population, now used to describe something bad.

Consider, as a non native speaker, if English speakers used your language of origin to mean "terrible". I don't know your language, but imagine it was Latin, and kids would taunt each other saying, "ha ha you're so latin" on the playground, and "windows search is simply latin". This is a constructed example, of course, but the issue is not that the word is offensive (fucked, or shit would have been fine), but that the word derives its power from being linked to marginalizing and othering people.

For more:

https://www.r-word.org/default.aspx?viewfullsite=Y

Thanks, I didn't know that. I'm Polish, and, of course, we have such words; not even swear-words per se, just words consistently used over time in a derogatory manner, while also being associated with a group of people. We also have a very insulting word for mentally disabled people, equivalent to the English word "retard", but it functions only as a noun (so no direct equivalent of the "retarded" word) and applies only to people. It's interesting because both noun and adjective versions of a word for homosexual people exist, just like in English, and the same is true for many other insults, especially ethnicity and sex-based ones.

Anyway, I agree that such words should be avoided and not used. (Although it's hard to avoid it completely when talking about the word, without using it.)

Please don't treat pfooti's comment too seriously though, most people would not react to the way the original poster used the word and would disagree with the purported disrespect the usage implies according to pfooti.

The world is a big place and everyone does not (thankfully) live in the liberal US coastal areas :-)

Speaking objectively, I linked to an appeal from a broad coalition of disability rights activists to stop using a particular term in the vernacular that has a demonstrable link to the othering. This is not just me, nor is it "liberal coastal areas".

A little more concretely: it's not up to the person saying anything to decide whether or not the thing they say conveys disrespect or harm. It is up to the people harmed by those words to decide. You can, of course, choose to ignore the harm done and continue on as before.

That said, I'm sure the OP's intent was innocent enough - this is the kind of thoughtless use of the word that I (and many other people) work to problematize. I had intended to do so publicly but without much fanfare (one quick sentence), to draw attention to this fact in hopes that we could all work to shift the language.

Speaking subjectively, I find it troubling that the notion that basic decency toward other humans who have a relatively simple ask (use "terrible" or "garbagefire" or "shitty" instead of "r_") is viewed as a "liberal US coastal" practice that should be avoided out of sheer tribalism. Not sure there's much else to say rather than "troubling".

This whole thread is OT at this point, but I'm simply not willing to let this go. The lexicon has plenty of synonyms for "is terrible and unintelligently designed and frustrating to use" that we can afford to drop one.