I am not conversant with any of the research on differentiated instruction, but I have my own experiences to draw upon in discussing it. I tend to think that good teachers have always used differentiated instruction, whether or not they have had a name for it or incorporated it into formal lesson plans. No matter what, we must meet each student where he or she is, and that is the basis of the whole concept, applicable to the gifted child, the disabled child, or the child in the middle of the bell-shaped curve. I have taken note that the idea of the visual, audio, or kinesthetic learner has been largely discounted, rather recently, I believe. However, I don't think that particularly matters. The more ways we can connect with our students, the better off they are.
What concerns me about differentiated instruction in the past several years is that the pressure to teach to the test and to make teachers accountable for learning in only a one-dimensional way have completely superseded differentiated instruction, as well as many other sound pedagogical ideas. There is no longer a drive to reach the student as an individual, only as a delivery system for satisfactory scores. I am fortunate in not teaching where this is an issue for me, but I listen to my friends who are subjected to all of this. Who has time for differentiated instruction when most of the school day is spent on test drills?
So, I don't think it is a fad, but I do think that it does not have the importance and respect it deserves right now. And all I can do is hope that we can find a way to help American education transcend American politics.