I have to say, I really loved Voice Thread. I can imagine creating a very concise introduction/explanation of the concepts to be discussed in class, spreading it across several slides, and then having students post their questions on the appropriate slide. Not only would that make our in-class time more targeted at their concerns, but it would also be a useful exercise for them and encourage some reflection (because they would have to spend a little more time formulating the question and figuring out where it fits into the explanation provided).
I also think the response options (text, video, or audio) are great. As someone who expresses herself much more effectively through writing and absorbs information much better visually, I appreciate that discussion participants are able to choose the medium best suited to their communicative and learning styles. I would definitely use Voice Thread even for classes that weren’t online.