Rich text in the modern world is almost exclusively solved by using markdown because it’s such a trivial solution.
citation needed
markdown is not a trivial solution: there are many different implementations, it’s a barrier for non technical people and it allows you to embed any html, so you need an additional html sanitizer.
my definition of a “rich textbox” is a WYSIWYG field, and markdown does not help you with this?!
yes, you probably would not save the formatted text normalized over multiple database columns, and only use a single field for a the text with formatting embedded in html or another format, and another one with the text without formatting for possible full text search. but even if you would solve this using markdown (which limits you to a quite small subset of text formatting and bad extensibility) you would still need a good data format to store the formatted text in memory that allows you to render the text. and markdown does not help you with this either?!
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…