"What martial arts focus on x?" -style questions
Where x is "grappling" or "self-defense" or "high kicks" or "joint manipulation" seem perfectly valid to me. Some arts do focus on certain areas more than others. It seems plausible that the answers to these types of questions could provide valid, factual responses beyond mere opinion or anecdote.
And if simple opinions of the style "my art is best" are offered, then it seems natural to avoid voting these up or to vote them down if they don't offer clear references as to why these arts are suited for the given requirement.
Another approach is not to recommend individual arts, but instead to explain how to evaluate arts and schools for whatever requirement is given. In the closed "What martial arts focus on self-defense?" question, I attempted to provide an answer that didn't champion a particular art, but rather offered advice about what elements make a school better set up for teaching self-defense.
"What martial arts are suitable for y" -style questions
Where y is "blind and partially sighted people" or "children under five" or "amputees" are equally valid in my opinion. Again, this is likely to solicit opinion, but provided that opinions are backed by solid explanations, they're likely to offer useful information for the asker and future visitors.
Avoiding the word "best"
I think the key might be to avoid loaded words like "best" that will encourage arguments. Questions adopting this format should be edited to use "suitable" and "focus on" instead.
Using "martial arts" instead of "martial art"
Again, I think it's less contentious to ask "what martial arts are suitable" than it is to say "what martial art is suitable...". The plural form shows that the asker accepts that there's not necessarily one "best" art and is looking for three or four to evaluate. It may also be less likely to encourage "my art is best" -style responses.
Choosing a martial art is difficult; we can help
Finally, choosing a martial art to study is a common question because it's very difficult for beginners to evaluate individual arts and schools. There is no site on the web presently that attempts to do this from a neutral standpoint. I think we should accept it as a challenge and do our best to help.
It might be valuable to have a "How to choose a martial art and martial arts school" wiki for a general overview. But I don't think it worth closing more specific questions about suitability for a particular area or person -- instead, we can simply rewrite questions where necessary to solicit facts rather than opinion.