"We need some way of commonly defining what problem we are talking about."

The problem we are talking about is people being discriminated against and excluded because they are different.

The only reason labels are problematic is because they are used in the context of this kind of dicrimination. "Quaker" and "Methodist" both started out as intentionally insulting labels. "Cretin" started out as a supportive label.

Learning disability seems, for the present a reasonably satisfactory label, insofar as any label at all is appropriate or necessary, which it very rarely is. The fact that it covers an enormously wide range of abilities and disabilities is ok. It makes it harder to use as an insult. "Label jars, not people."

The relevant question about an individual should never be "do they fall into this pre-determined category?" but "what needs to be done in order to make sure that this person can get by?" And much of the time when you ask it that way, it's not so much the person that needs to change, it's the people and things around them.