100% agree on both fronts. The shared tag concept would be similar to Evernote's shared Notebooks functionality.
A very common use-case: I would like to share development/education notes between my work and home computers. Since I use Bear for everything (journaling, personal finance, etc), I absolutely do not want my full personal account to live on a work machine, but would like to push work-related notes to my personal account without having to manually export or duplicate.
I'm loving Bear, but will have to hold onto Evernote before totally committing. I need to be able to share tags (in a notebookish, folderish way). For instance, recipes. I have a large, shared recipe notebook in Evernote that is shared with a lot of folks.
I need shared notebooks too! Evernote shared notebooks were great for organizing information for teams and were often my deliverable for entire projects. Its a great way to curate a list of tags and make a mini database for a specific topic/project/use case. Lets get this and then I could use it as a proper business tool. Tempted to go back to Evernote if it doesn't happen...