Why are some tasks more difficult to learn than others? Hoffman et al. (Accelerated expertise: training for high proficiency in a complex world. Psychology Press, New York, 2014) hypothesized that certain task characteristics—termed “dimensions of difficulty”—hindered learning and performance. Previously, we tested two dimensions: consistent vs. variably mapped and static vs. dynamic. Here, we test three more dimensions of difficulty: sequential vs. simultaneous, discrete vs. continuous, and separable vs. interactive. In each study, we manipulate a single task feature (dimension of difficulty) while holding all others constant. Tasks with continuous (rather than discrete) features slowed participants’ performance but did not impair learning. Learning and performance were unimpaired in tasks with interactive (rather than largely separable) processes. By contrast, we found strong evidence that simultaneous tasks (i.e., those that demand multitasking) inhibit learning, slow performance, and increase task errors. Importantly, this occurred in the absence of perceptual and mechanical bottlenecks present in most other studies of multitasking. We also are the first to examine simultaneity on learning a new task while controlling for other dimensions of difficulty. We discuss the potential impact of these results on current theory and application to real-world domains.