Specifically, the idea of "crazy = bad" being an ableist concept.
First off, since this is a public post, I should mention that I have been told by multiple people that I cannot lay claim to really being mentally ill or disabled in any way. On the other hand, it would be transparently false to say that I'm neurotypical: I have issues with depression and ADHD that have plagued me since I was a small child, and my thought processes are not what society deems "normal." So while I'm not truly disabled or mentally ill and can't speak for those who are, I do have something of a glimpse into their world.
Second, this is my perspective alone. Don't use it to justify ableist behavior. I'm one person. My opinion is mine alone. This is about my thoughts, not your actions.
That said, I'm really bothered by the idea that we should stop using the words "crazy" and "insane" in a negative context because it's offensive to the mentally ill. I agree that we should be more careful about how we use them--no calling someone crazy just because they disagree with you or did something that pissed you off, for example--but I don't like the implication that craziness and insanity are value-neutral states which should be embraced as part of the colorful rainbow of the human mental spectrum.
Being mentally ill to the point of insanity means you have lost control of your own mind. It means you can't function. I cannot speak for mentally ill people, but when my depression and ADHD get bad enough that I can't translate my thoughts into the actions I desire, I don't want sensitive language and respect for my differences. I want treatment that will make it better.
Now, when I'm just being weird, when I'm bouncing off walls because I'm in the grip of some exciting thought or making strange noises because something has me wound up, I'll be bothered if you get on my case to "stop being a freak," because things like that don't interfere with my actual functioning--they're just looked down upon because they're not "normal." And therein lies the issue.
The problem isn't that "crazy" and "insane" have negative connotations, because the concept they describe--a lack of control over one's own thoughts, an inability to function--is a negative one. The problem is that they're intertwined with concepts of abnormality, of simply being weird, and that that has a negative connotation. The problem is in the way mentally ill people are treated as a whole, and yes, being more careful about the way you use certain words will help with that, it's true. But pretending that it's a-okay to be "insane" isn't doing anyone any favors.
As someone with a toehold in the world of mental illness, I want to see a lot of change in the way society treats the mentally ill. I want to not be shamed or criticized for needing pills to function; I want my quirks and harmless odd mannerisms to be accepted. I want to get rid of the stigma. But--and I should emphasize that this is just my opinion--I don't want to erase the negative connotations of the concept of insanity.
Which brings us back to word usage. Sometimes, it's actually quite easy to clean out the ugly reminders of the kyriarchy in your language. "Gay" and "lame" can be easily replaced with "uncool." "Retarded" can be avoided; if you want the punch of the word without hurting mentally disabled people, just swear and use the more neutral "stupid" instead.
But "stupid" and "crazy" aren't simply pejoratives that can be easily replaced; they describe fundamental negative concepts.
Stupid. Poorly thought-out, intellectually shallow, lacking in understanding. You can use those terms instead, sure, but they still denote a lack of intelligence and consideration. Do we avoid calling people out on doing things that they didn't think through properly because to do so we have to denote intelligence as "good" and lack of it as "bad"? Seriously, I don't know--but I don't think it's as simple as, "oh, just don't use the word 'stupid,'" because it's not simply a word that can be easily replaced by another, the way "gay" and "lame" can, it's a core concept that is negative for reasons beyond prejudice and oppression.
The same goes for crazy. Irrational, potentially dangerous, out of control, incomprehensible or logically inconsistent--or all of the above. Something being "crazy" in that sense means it is genuinely a problem. Even if we don't use the word "crazy" to describe it, we're still returning to the idea that lacking internal consistency and self-control is a bad thing--the concept described by "crazy." While the stigma on the mentally ill is a product of the kyriarchy and needs to be eliminated, the concept of "crazy" as a negative, dangerous thing is a valuable part of our vocabulary. How do we disentangle these two things? Is it really as simple as "don't use the word 'crazy'"?
I don't know whether or not we should be eliminating the word from our discourse, but I do know that for those with mental health problems, turning "insane" into a neutral concept isn't the answer. I'm aware, of course, that people who suggest avoiding the word aren't really thinking, "Oh, it's okay for the mentally ill to not get treatment, because it's all right to be insane." But sometimes that's how it comes across to me.