Looking at a classroom that is running in Harkness mode, it is easy to see that many of the best practices we are supposed to implement are happening. The classroom is student-centered. The students are highly engaged. The students are taking responsibility for their own learning. However, one thing that people perceive as missing is differentiation. "You mean that they do this discussion thing every day? What about the kids who have trouble learning this way?"
One of the things I used to question a few years ago was how to structure a lecture in such a way that I would be able to reach the variety of kids in my classroom. And honestly I used to pass off the idea of making that happen as impossible. Over the course of the week I might have been able to teach one day or one part of one day for each of the kids, but to try to reach each of the kids every day during every lesson was simply not an option. So, I relegated the idea where I normally relegated all such ideas: into the category of "educational theory that isn't in touch with reality".
However, with Harkness, what I have found is that the differentiation all but takes care of itself. Early in the trimester, it is fairly common for the kids to ask if it's ok for them to attempt to solve an exercise a certain way. "Am I allowed to draw a picture for this?" "I found a formula...can I use it?" But after a few days of hearing me say, "Yep, if you think a picture will help," or "As long as you can explain where the formula comes from,", the kids begin to figure out that they can use pretty much whatever method they can devise and understand. Slowly they begin to take responsibility for their own learning, and once that happens they begin to figure out how they learn best. Does drawing a picture help, or do the equations make more sense? Or is it some combination of the two...or something else that works best? Because they aren't being told how to solve the exercises, they tend to head for their comfort zone rather than doing what they're told, and in that the instruction is differentiated. Granted, the kids who are stuck in memorization mode don't reach this point as quickly since they are spending their time desperately looking for someone to imitate rather than working through the material themselves. But even in that search these kids tend to find which of the other students attacks the exercises in a way they understand, and in that the instruction is differentiated.
So yes, we do this discussion thing every day. And it's the most differentiated the instruction has ever been in my classroom.