Is 'all' better than the current 'default', or will we have both 'all' and 'default'? default (currently) means the default website all means for all websites
Definitely the all functionality can be rolled into default, but that might be confusing. I personally would prefer the separate version, it allows treating the default site as any other.
How about the simpler:
sites/all/javascript sites/all/stylesheets sites/all/images sure, it makes sense
* */vendor is a worthy contender to consider - this way we won't mix drupal and non-drupal code too much - this will be easier to upgrade but otherwise I don't have an idea on all implications
I don't see the point of 'vendor'. Well, the only point I see is not to mix drupal and non-drupal code. The equivalent in the linux world will be /opt and /usr/local - you put there things that are not coming with the distribution, so you are responsible for their upgrades and maintenance.
For example: the various Javascript based editors - they don't come with drupal, only their integration modules.