I think that some of the concern does revolve around Job's departure. Its hard to imagine him giving up the CEO position unless he's pretty sick, and thus unlikely to be involved in a major way going forward.
In the business world it is a common perception that "big companies don't innovate". So what has made Apple an exception? I suspect the answer is that Apple doesn't innovate, it is instead a facility for implementing Job's creativity. This point of view is supported by Job's unparalleled creativity, his obsessive involvement in details, and his autocratic management style. What other tech company has had a CEO with 313 US patents and 80 published patent applications?
A discounted cash flow analysis suggests the market believes that there are two or three years of growth left in Apple that will arise from existing products. Whether this is fair, I can't say.