Right now all I know is post constructively, don’t get moderated for violating forum Code of Conduct, and like constructive posts by other users.
No, I would have seen it be enforced by moderators now, the block is just a safety feature to kinda keep Blizzard from being held liable if someone posted a link to a malicious website. Of course any URLs you manually list must still be be in compliance with forum Code of Conduct.
According to the post on the older forums by Community Manager Josh Engen, anyone should be able to earn the privileges through constructive posting. However, I have no details if this means they privileges are manually granted or not.
Players who contribute to the forums in a constructive way will be granted additional features and permissions such as link previews, images, gifs, and embedded video posting.
As I said there is no Community MVP program for Overwatch yet, this may change, and if the Overwatch Community Team decides to start one, I hope to be a part of it along with my current group (the Blizzard Customer Service and Tech Support MVP group).
As far as constructive posts, here is content that I feel meets and exceeds this definition and I wish to see other users to create such content (yeah some its mine):
The other active MVP user from my group who posts in Overwatch is Narull. There is about 30 of us in our group, but most of them are focused on WoW, SCII, and D3.