I agree with you that an application needs discoverability. Like a text processor for a writer or a photo editing software for a photographer. It should make sense for a particular domain, requiring only minimal training for the medium. But we can't abstract away the notion of computers. An analogy is car. Driving a car only require a minimal interface. Racing with one, however, requires a lot more knowledge. Repairing a car is where knowing the internals is the minimum. Different needs, different levels of knowledge. Most people would be happy with the iPad if only the applications were a tad more useful (File sharing is aweful). But some people do "general computing" and graphical interfaces usually fail in some way or another (especially with automation).