Here’s the missing feature that makes the iSlate the hot seller of 2010: iTunes subscription service. In validating the hardware, I lost track of the other half of the argument. The ultimate portable entertainment device is lost without service.

In the tablet/reader market, Amazon’s Kindle leads the pack for technology and service, now with B&N nipping at their heels with the Nook. The Kindle ships with always-on wireless service that provides a portal to Amazon’s e-book storefront, making the Kindle a library on demand instead of just a storage device. Killing the Kindle is relatively easy: add a comparable reading experience with rich data and multimedia features, top it off with Apple’s spit and polish, and bring the price below the value multiplier of the additional features and suddenly the Kindle and the Nook look overpriced niche-market dinosaurs. Amazon doesn’t have iTunes’ cachet with digital media, so their leg up just got broken. Besides, digital movies are faster sell and more established paradigm than e-books, so Apple can play the movie/music card first and spend the next two years catching up on literature.

In the movie rental industry, there’s but one player to discuss: Netflix. As their on-demand service quietly (or not so quietly. Thanks to relentless pop-under ads, Netflix is my least favorite favorite movie service) grows, Apple has been chipping out their own hold with downloadable movie rentals in the iTunes store. Up until now, Netflix has Apple beat: at $4 a pop, “renting” movies from iTunes with their inconvenient time limitations fails dismally compared to Netflix’s subscription and on-demand services. But, if Apple were to introduce a device with HD capability, always-on wireless access, a great screen with HDMI/projector connectivity, and a flat-fee portal to iTunes, they’ve addressed all the advantages of Netflix, plus one: you get the hardware to boot. Your DVD player just became obsolete, and your Tivo is starting to look a little bulky.

There’s no reason Apple can’t do this. In fact, Amazon blazed the trail by demonstrating that a “free” dedicated 3G portal to content is profitable, and that people will pay nearly $300 for a device that does little more than poorly imitate a book. (No offense to the Kindle, but paper is still better looking than e-paper.) As another of my brilliant friends pointed out, boarding a plane with an iPhone, a tablet and a laptop is a little cumbersome, but compared to carrying an iPhone, a laptop, three magazines and a book, it’s delightful. (He then goes on to add that the tablet should also run a limited range of desktop apps, like the iWork and iLife suites. Preferably apps that Apple controls, so they can manage the processor load until they can get the device fast enough to support the likes of Photoshop and your favorite games. I concur.)

While iTunes doesn’t yet offer books, newspapers or magazines, they do offer movies, music and applications, and with iTunes Apple managed to change the landscape of digital media. Why wouldn’t every print publisher rush to resurrect their fading profit stream through a well-proven media distribution service?

Now, can someone explain to me why Apple really put an SD card slot in their latest MacBooks?