It would be nice to be able to actually markup hypertext... but that still appears to be outside the range of feasibility.
Maybe it could be called TLM - Text Language Markup?
It turns out the given name is Wave, and is a dialect of XML still being tweaked by the wizards of Google and a few thousand developers who just saw it for the first time this week. I spent the time to see the demo, and have been following up to see what others think. The gut level reaction seems to be one of hope that this is something good, because it does seem to be a game changer.
The void being filled here is hard to describe, I've been trying for years... it all boils down to context. When you send an email and then reply, you're forced to use all sorts of mechanisms and tools to attempt to keep your train of thought, your conversational cache, your context. Every step away from being able to just add a note, circle something, highlight or annotate text to draw attention makes conversation less efficient. Wave is going to provide a mechanism that does a much better job of preserving context.
I expect Wave to succeed because of the good Kharma that Google has built up, along with their pledge to open-source most of it... which will greatly help adoption as a defacto standard.
The money quote for me was Tim O' Reilly's mention of the need for granularity when editing book manuscripts, which I feel vindicates some of my howling in the wilderness these past years.