Everyone's posted good reasons, but the REAL secret that makes Mac so great is this (and I used to work at Apple, I know of what I speak):
The entire company -- from the CEO on down -- is incredibly passionate about making products that CUSTOMERS will absolutely LOVE. It is what gets them out of bed every morning.
It's not actually always a formula for success, and many of the factors mentioned above (like the hardware/software integration) have helped make sure that the company is successful rather than just "great stuff but no money" as many other passionate companies often end up.
Too many other companies in this industry are geared towards serving their stockholders or advertisers and they take the customer too much for granted. Apple is not one of those companies.
Thanks to people like Steve Jobs (but not BY ANY MEANS limited to just him), there is a LOT of emotion in Apple products. People can look at this stuff and SEE as well as FEEL that the people who made it poured their heart and soul into it. That it is a product of passion that sweats even the smallest, unseen details. Look at the BOX most Apple stuff comes in, fer pete's sake!
Not every idea out of Apple is a great one, or a sustainable one, and it is one of the few tech companies willing to take real chances and fall on their face once in a while, which is another part of what makes them so great actually. But enough of their ideas are genuinely, insanely great, so much so that they routinely change the way the entire world works, generally for the better.
Apple's goal isn't to be the biggest company in the world, though they may well end up there. They want to be the company that made awesome stuff, and I think they do get a lot of "fuel" out of seeing how much people use and love their stuff. Facebook employees constantly tell me much the same thing. While I think FB is a very different company, I think Zuckerberg, like Jobs, is driven by a lot more than just seeing how rich he can get.
Anyway, that's what makes Apple so awesome. It's what drives every decision -- even the ones we don't always immediately understand or gnash our teeth about, even the ones Apple ends up being wrong about. It's why the hardware and software will always be integrated, why Mac OS X is the way it is, why Apple's customer service is legendary even by the extra-picky standards of Mac users -- because even when we don't immediately see it, we sense that SOMEONE at Apple staked their reputation and livelihood on how strongly they believed they were right.
Apple is a company technologists who remember that art and beauty are what make life worth living.