Oh, interesting. I don't use the "!" in tags, so I hadn't thought to check that out. I see what you mean; it looks like it's perfectly okay to use in one's own tags, and it works as a sort/search term within one person's tags if you click on it, but for the overall system (either in subscriptions, or clicking "all" at the top of search/sort page), it works more as a plus sign, giving you bookmarks that contain both "slave" and "sheppard", but not a single "slave!sheppard".

Which is... kinda freaky.

It looks like using a slash, either forward or back, is the best way of picking up anything tagged with a !; "slave\john" and "slave/john" both pull up identical lists, which include bookmarks tagged "slave!john". They just also pull up anything with both "john" and "slave" in the tags somewhere.

I'll poke around more and see if I can come up with anything else that will work, hopefully more efficiently.