1: Industrial design, especially aesthetically.
2: Aesthetics and certain experiential aspects of User Interface design (note: I do *not* think they are universally great at UI design, but they do some things very well, and they get the visual aspect for sure; this ties into #1 as well as #3).
3: Marketing, particularly "lifestyle marketing".
Those 3 things combine to allow them to succeed at the rest of the things they do, which then simply further cements their profits and success. E.g. Sony was unable to create the kind of format lock-in they created with iPod and Apple because A: they did not have the entire "ecosystem" (no iTunes), and B: they did not just push one format, they mandated it, whereas Apple's approach is "our way is the right way, but we're compatible with *input* from others" (read: mp3 support on iPod, PC file cross-compatibility, but *not* allowing iPod sync with other apps besides iTunes, or allowing iTunes to sync with other hardware). With this "strict but 'open'" approach, they are able to assimilate users on other platforms easily and comfortably, while making it difficult or impossible to go the other direction if you ever want to. All roads lead to Apple in other words. It's all primarily driven by their "underdog" status and the perception of "cool", which is supported heavily by their visual design.