Replying to jtauber's thoughts on the subject;
I think that the system for replying to a thought should be very straightforward; just have a bi-directional link, and perhaps an (optional) notification. Similar to how I can extract a card from my own stream and create a new stream, I should be able to extract a card from someone else's.
Deleting the reciprocal link would be a nice feature, but I do think it can wait until there is at least one instance where somebody wants to do it.
Alternately, it might be interesting to just send a notification to the respondee alerting them to the presence of the responder's new stream, and not add it by default. The link from the responder's card to the respondee's stream would be automatic, but the need for action on the part of the respondee in order to hook up the discussion would allow for a more curated discussion and would allow heated exchanges to proceed more deliberately than the usual commenter pile-on.
Despite the fact that the community is quite small right now, and is unlikely to have discussions where this sort of mitigation is necessary for quite a while, this design would seem to fit better with thoughtstreams' existing design for more deliberate, thoughtful presentation of ideas; similar to the way that cards right now require explicit action to publish and everything begins as a draft.
It would be nicer if the preview "button" were instead a live side-by-side view of the rendered markdown, a-la stack overflow. (Except make it responsive, so if I have a full-screen window with scads of white space on the left and right, I can glance over to the left or right rather than scrolling down.)