A good proposed feature!
1. It would be great, if we could collect some pictures. 2. I proposed that the tag is allowed also for highway=steps
This proposal is still classified as draft, but it seems matured enough to be worth commenting.
Unfortunately, it heads into a wrong direction. There are already plenty of keys (surface=*, trail_visibility=*, smoothness=*, obstacle=*, sac_scale=*, mtb_scale=*, uiaa_scale=*, width=*, incline=*, disused=*, abandoned=*, wheelchair=* etc.) describing various aspects of paths. Additionally, route relations may indicate the importance and usability of a path. There's no need to add one more key whose sole pupose is to give an overall score. This kind of scoring should be left over to the applications. The reason why the Mapnik layer doesn't differenciate paths is just that the autocrats empowered to configure it are not willing (or too incompetent) to do so. Inventing more keys won't change that situation.
The only part of the suggested pathtype definitions which cannot currently be expressed by other tags is the usage frequency. This may be worth a new key. It would be applicable to other highway=* too, up to motorways. --Fkv (talk) 22:25, 8 November 2013 (UTC)