*Preparation. The student comes to class with an idea already formulated. Not always possible, because the discussion might not be going in that direction.
*Relevance. The comment relates directly to other things said; it is part of the conversation. The student doesn't always make the same comment about everything. Relevant, improvised comments are even better than prepared statements. You can also prepare to improvise.
*Thoughtfulness. The student might not say everything that occurs to him/her, but will self-censor to a certain extent. Too much, and he/she will never say anything. Not enough, and you have the glibness syndrome. We encourage glibness when we ask for participation but don't necessarily have a structure that produces good student response.
The classroom discussion is a collaborative enterprise. The best discussion is not one to which you could go back, listen to the tape, and judge everything to have been perfect. Rather, it is one in which students responded to one another, developed ideas suggested by others and by the profe. There will be digresssions and even irrelevancies. You would have to go back and edit it to make even the best discussion a well developed essay, but that's not really the point, is it? It should be more jam session than recording session