One of the difficulties with disabilities is that they are not as black and white as gender / race etc. - e.g. in race terms you are usually considered to be from a specific group...
With a disability there is very often a scale of severity - e.g. eyesight as a nice non-controversial discussion, there are millions of people (I am one) who are short or long-sighted, yet would not consider themselves to be disabled, or to have a disability - yet at an extreme someone who is blind clearly does...
A more difficult one for many is the discussion above reference Aspergers / Autism etc. - one definition in the sector is that it is the essence of maleness, and a scale on which we all fall, and at one end it is 'socially conflicting' (i.e. Autism - doesn't sit with how society works), towards the other it is 'socially uncomfortable' (i.e. Aspergers - can sit within society, but walks a different path), and elements of each are found in many / most people, and esp. male...
Once you start to put discussions into that context - that there is a scale and sometimes no polarisation, it becomes very difficult for a forum to manage conversations... There will be a number of views around and many of them are valid (but may still contradict), and some are clearly invalid - the difficulty in moderation is walking the tight-rope through those different views and deciding what is or isn't acceptable.
Just because two people don't agree, doesn't make one of them wrong, doesn't make one of them disablist... Equally, defining where the cut-off is, where comments are acceptable or not is not a simple black and white decision, it is too fuzzy a topic (in logic terms), and therefore the decision making needs to be the same...
A part of the problem we have though is that this is the written word with no other signals, so how one person sees it will be different to another, and a member on here who is involved / a part of the world which understands a specific disability (for themselves, or as a carer) will have a different perspective to staff who run an online forum.
And as mentioned above there are those who look at the world in black and white with no emotion - and equally many who don't - while we don't want people bullying or setting out to oppress, equally we live in a country where free speech is a key part of our society, so we can't allow people to close down conversations just because in their view it is not appropriate - a very difficult balance for MNHQ.
I think that the underlying approach needs to not be trying to define whether something is disablist or not - but a more generic question as to whether a comment is setting out to help or hurt, enable or hinder - is it supporting and improving the thread, or is it causing aggro - and I suppose that fundamentally that is what Mumsnet has in its talk guidelines...