Apple is a trend. What keeps Apple going is it's high profit margin. People are still content on over paying for Apple products for the brand name and Apple is successfully keeping it's part costs down by producing a single product line per market. (iPhone, iPad, iBook) The consequence being, Apple is putting all it's eggs in one basket. Each iteration of next generation iPhones, iPads, iBooks and etc are practically the same. Apple is no longer creating revolutionary products ever since iTunes came out. While successful now, it could quickly change if a competitor were to gain a differentiation advantage much like how Apple took over the mp3 player market.
Due to it's popularity, it unfortunately means the stock is over bought and prices are sky high. (4 to 1 price to book value) If anything were to happen to that profit margin for a quarter (new out sourcing laws, fashion trends, competition) you could see Apple stock drop 75%.