Coelocanth: Thing is, when the word is used as an insult it
is a slur. Saying "That's just retarded" is a slur because the root of that comment/insult goes right back to comparing it to the mentally handicapped.
When the word is used properly, it's
not a slur. Such as "Progress was retarded by this poor design choice".
But used in the context of saying something is basically stupid or unintelligent isn't using the word properly.
This.
I also used the word "behindert" (German for "retarded") as a teenager a lot, both to insult people that I considered idiots and pretty much anything else I considered stupid - it was used a lot this way where I grew up. But really, when I used that word in the presence of a friend who has a sister with a mental disease I just got ashamed of myself and gave more thought to why it's wrong to use that word this way.
Of course the word in itself is not a slur but when you call a mentally healthy person a retard you insult not only that person, you insult people who actually do have that kind of illness - after all, this use suggests that this condition is something worth being ashamed of, something you should feel bad about, something that makes them lesser beings. But I do respect people who have illnesses of this kind, I would never make fun of them for this or consider them a strain on society - so it is wrong to use the word as an insult and that's why I don't do it and why I hate it when other people do.
And if a word gets such a negative connotation it's not the fault of some overly sensitive folks, it's the fault of those people who established it as an insult by using it this way. Those people are to blame, not the ones who do not tolerate this use.