All great use cases. In the case of a global commenting system, I think that rather than "everybody agreeing on one standard," there's an opportunity for one major player to define the standard simply by announcing support for a specific format, and implementing it into their product. Disqus would be a good candidate.

Anyone who wanted their tweets picked up by the hypothetical Disqus engine would need to annotate accordingly, third parties would build apps to support this annotation, and the cycle would perpetuate with everyone conforming to said standard.