i think you mix two concepts here:

the first is basically that you can post to multiple communities at once (community = tag)

the second is that users can assign tags/communities to existing posts, and vote for them

i like both and already thought about the first one, but not the second one… I first thought it might be a problem that if you add a community/tag to an existing post it will be immediately visible on said community/tag, but if this is a problem for said community, someone could down-vote there and it would vanish. but then you would need two up votes: one for community fit, and one for the post itself. could be a problem, if you post one link to a very popular community and a very small one, then the post would get many upvotes and be on the top of the small one…