As you brainstorm some of the problems that might occur if differentiated instructional strategies are not used when planning and implementing adult learning situations, I believe firmly that adults, like other levels of learners, must receive an education that aligns with their preferred learning styles, pace, and academic levels (skills mastered, skills lacking), etc. If differentiation is not applied, adult learners will turn off, become bored if it's too easy, overwhelmed if too difficult, so teachers must assess each learner's level of ability as well as unique learning idiosyncrasies to ensure that instruction is tailored to meet his or her academic, social, and emotional needs.
One great resource that defines DI comes from this source:
Tobin, R. (2008). CONUNDRUMS IN THE DIFFERENTIATED LITERACY CLASSROOM. Reading Improvement, 45(4), 159-169.
This solution examines what happens if differentiated instructional strategies are not used when planning and implementing adult learning situations. This solution is approximately 500 words and includes three APA-format references.