Twine looks like fun to me, the standards support looks great, and I've applied to be a beta tester. Still, one point that Shelley Powers made about it bears repeating and rereading:

To me, the semantic web means the web in the wild, not centralized in a specific tool or environment. If this becomes a "Facebook and Wikipedia mashup", it might be successful, and it might be semantic, but it isn't the web. The whole point of the semantic web technologies is for each of us to annotate our data, wherever we are, regardless of tool, and begin to really drive out the tiny threads of true meaning on a global scale. If we have to leave our places where we're at and go elsewhere, this seems to create a disconnect, right from the start. I have this same quibble with the other 'mainstream applications using semantic web technologies', so the concern isn't targeted specifically at Twine.