"Cool" is a completely subjective thing that I would suggest has little to do with "technical" competence.
I tend to think there are a few factors that generally are associated with any product being cool.
1/ It tends to be premium priced for it's general market.
2/ It tends to be limited in numbers compared to "mainstream" competitors.
3/ It tends to have an attention to detail in the styling
4/ On occasion it can be intentionally "over engineered" for the task it's designed for
There are a few other points but that pretty much covers the jist. People like to feel special or differentiated from everyone else and some consider a Mac as "cool" for just that reason. A £3000 Omega watch doesn't tell the time any better (generaly) than a £30 Casio, however it's seen as aspirational because not everyone has one, it's expensive, looks good, not everyone has one, it's over engineered, and, not everyone has one

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Now sure, just like an Omega, there are people that either need a Mac for a specific reason (print publishing and media is a great example), or just sees it as a tool to do a job and happens to use an Apple PC rather than Sony, HP or anyone else. For others they may have used DOS/Windows PCs at work or for a long time and "just fancy something different for a change".
Others will buy a Mac because they see it as cool, or aspirational. The slightly more ugly side is they buy a £1200 Mac book to flout "they can" rather than a "cheap, nasty £400 laptop". The same goes for "exclusive" cars, watches etc.
Technological capabilities really don't come into it. The straw man that a decent Windows 7 PC is insecure, unstable or bloated is out of date by several years.
The two OSs are broadly comparable in terms of capability, both having strengths and weaknesses (Beach ball vs doughnut of doom anyone?

)
Apple produce some cool and stylish kit, they specialise in elegant industrial design. Apple don't license the OS out to PC builders for exactly this reason. They have a niche at the premium end of the market and don't want to see the "brand" devalued by being installed on "adequate" PCs at the low end of the market.
The lack of success of Linux on the desktop, even preinstalled and configured by people like Dell, is a great example of why it has very little to do with the OS and much more to do with the desirability of the whole package.
That's why the Mac is unlikely to become "uncool" for the foreseeable future as long as Apple refuses to go mainstream.
They need to be careful though.
When the iPhone came out it was hugely desirable and "cool". Now, although commanding a premium price and still looking stylish (although I would argue the 5 x 4 home screen and UI is starting to look dated after 4 years) you'd be hard pushed to find anyone who on seeing a iPhone thought it was "cool". It's become ubiquitous and lost it's mystique.
Just my 2c...