Absolutely agree on the 'spectrum' point, and I would argue that it's not that case that 'normal' people
can be somewhere on it, but everyone is on it, even if they're all the way to the left.
If we take OCD as an example, there's clearly a huge range from 'I like the TV volume to be set to an even number' through to massive impairments to functioning to the point it's impossible to hold down a job, or that it's damaging your personal relationships.
I am a bit less comfortable with saying that it's "super subjective" where you draw the cutoff though. I agree there's some subjectivity there, but the diagnostic cutoff (per DSM-V) is quite clearly defined, and leaves relatively little room for subjective interpretation: "The obsessions or compulsions are time-consuming (e.g. - take more than 1 hour per day) or cause clinically significant distress or impairment in social, occupational, or other important areas of functioning."
I think it's unhelpful if folks who (for example) like their TV to be set to an even number talk about 'their OCD'. Or people who like things to be neat and tidy. Similarly for ADHD, the verbiage is: 'interferes with functioning or development/there is clear evidence that the symptoms interfere with, or reduce the quality of, social, academic, or occupational functioning.' Or ASD, where it's defined as 'caus[ing] clinically significant impairment in social, occupational, or other important areas of current functioning.'