My thoughts:
1. Requiring others to do the tagging will stifle new users. It creates a chicken/egg problem where I need my posts tagged to draw attention and gain followers, but I need people paying attention to get my posts tagged.
2. Trolls will troll. You'll get bullies who visit your profile and tag all your stuff as hate speech out of spite. The current flagging system needs a mod to actually suppress content, but it sounds like your proposal wouldn't.
Well right now #1 isn't an issue - at least not to nearly the same degree - because we can tag our own posts/profiles. People interested in calligraphy find you by searching related tags, but that wouldn't work if you had to wait for someone else to add those tags and you had no followers to do so.
StackOverflow's tags aren't used for moderation the same way as you're proposing. I can't really make it harder to find a question by maliciously tagging it, can I? (It's been a while)
@khird #1 is an issue regardless of tagging. If your a new user you have no followers, so you wont get many people who see what you post regardless. With that said, there is no reason to think you **need** tags to be seen. Many users would likely still view a feed that is inclusive of all posts and only **exclude** tags rather than use an exclusive feed and select **inclusive** tags. Ultimately, the power is with the user what they want to see. just as some users view an unfiltered federated fee now.
#2 Yes people will try. Any intelligently designed system will, however, prevent that. Stackoverflow uses the system I provided and because it is well designed prevents issue #2, not to mention prevents issue #1 with its designs as well.