Trello – Search for Text on Board

You could get this behavior right now by creating an organization and inviting everyone you want to be able to vote and comment to the organization, then creating a board within the organization and making your internal users the members of the board. Then go to board preferences and make the board visibile, votable, and commentable to organization members. It's a slight abuse of organizations, but it'll get you the behavior you want.

The side effect, right now, will be that the organization's existence, and its memberships, will be public-visible. Private orgs are in test right now (https://trello.com/card/board/private-organization-profile-option/4d5ea62fd76aa1136000000c/4e7206a8ec4fdc00003e63e7) so if the organization needs to be completely invisible to the public, you would want to wait for that.

I agree with your friend about not dividing up into separate boards by type of job.

With your examples such as 'Design ideas' etc, I think they really do fit naturally within different lists on the same Board. It's exactly the sort of usage that Joel demoed in multiple videos about Trello.

The only thing I can think of, to help manage very long lists, is to use Trello's Labels system to put categories on cards, and then use filtering. You should definitely give meaningful names to the default labels.

I wrote up some tips on how to use labels recently.

Having said that, fairly early on the Trello on Trello board got huge, and eventually they reduced the scope of it. So it's does seem to be a well-known problem.