As was mentioned, RE: the Rohling article, the idea is that depression in and of itself does not seem to cause/be associated with significant cognitive impairment. That is, folks with even moderate to severe depression have the potential to score within normal limits on cognitive tests, which is unlike what might be said for, say, Alzheimer's or Lewy Body Disease where there are verifiable cognitive deficits and associated neuropathology.
That being said, I do think the anhedonia, insomnia, and concentration/decision making difficulties are likely to be some of the more debilitating symptoms in a professional and social functioning sense, which is where I start to gather my definitions of disability. That is, regardless of what impairments there are associated with a condition (e.g., depression), disability doesn't come into play until there's an activity/participation limitation. This is one of the reasons I actually like the current WHO ICF material, particularly relative to what they had outlined in the previous iteration (caveat--I've never seen, let alone used the WHODAS). One example a supervisor has used previously is near-sightedness. Prior to the invention of corrective lenses, the condition would've been both a dysfunction/impairment at the level of body structure and function, and would have also resulted in disability due to impairments interacting with the surrounding world. Nowadays, however, the condition--while still associated with dysfunction at the level of body structure/function, generally doesn't result in much disability.
With depression, I can see how it could be a disabling condition, yes. I wouldn't say it's the norm, but I wouldn't say it's outside the realm of possibility, either. What the threshold of disability would be, for me, would entail determining if the symptoms of depression are directly and significantly impairing an individual's ability to participate in work-related activities. The sticky part, though, entails determining the threshold for the point at which that becomes compensable, and the stipulations of said compensation. Although that could all directly relate to the specifics of the condition itself. That is, someone with a "less-severe" severe depression may still be able to work part-time, while someone with a "more-severe" severe depression may not.
Edit: to speak to a point that (I think) was brought up earlier, I certainly don't think that because a condition is treatable/reversible that it's incapable of causing disability, and I would imagine pretty much everyone here would agree. Look at moderate head injury, for example. After a year or two, many/most folks are able to return to work, and it's not uncommon for this to be at a level commensurate with pre-injury functioning. However, in those first 3-6 months post-TBI, the condition is most definitely disabling. Same could be said for numerous other medical conditions (e.g., liver failure in a pre-transplant patient vs. functioning post-transplant; recovery from multiple compound orthopedic injuries following a motor vehicle collision; etc). Unfortunately, it just becomes a more complex situation with many mental health conditions, because the recovery trajectory is so varied, and it can be more difficult to directly measure/ascertain the degree of disability.
