The 90/10 rule is based on the years I have covered mobile technology, and the observations I've made of countless users of said technology. I've noticed that if a gadget doesn't grab the user's affection right from the OOBE, it has a good chance of being returned for another device that feels better while used. While the hardware design often attracts the buyer, it is the UX that keeps them.
I have noticed over the years that a typical mobile device user spends 90 percent of his/her time using just 10 percent of the device functions, thus my 90/10 rule. These handful of activities are conducted repeatedly during standard usage of a device, and how well it performs them is vital to make a user feel good about the gadget. These activities vary based on the type of device; a smartphone needs to do calls, email, text messaging and web browsing for example.
It is important for a manufacturer (hardware and platform) to correctly determine the 5 or 6 functions for a given device that the typical user will spend most of her time performing. Make these activities simple to perform, intuitive to a fault (no instructions required) and bullet-proof. That means these most-used activities never fail during standard usage.
The 90/10 rule is a large part of the formula that Apple has used for years, and it's garnered them an outstanding reputation with users. Apple engineers are masters at correctly determining which activities will be done by a typical user 90 percent of the time, and then making those tasks perfect. This is a big reason why Apple's customer loyalty is so high; buyers know the Apple gadget will do what they need it to do, do it well and right out of the box. Sure the more advanced tasks (remember cut and paste?) might be missing at first, but Apple correctly determines what matters and what can be added later.
Gadgets designed with the 90/10 rule in play are invariably perceived favorably by the new buyer. The device simply works as expected, without frustration and right from the get-go. The user feels it was a good purchase and will likely recommend the device to others as a result of the good UX.
You may have purchased a gadget in the past that wasn't built with the 90/10 rule, and lamented it from the beginning. Maybe the device would do sophisticated things well, but one of the activities you did regularly had problems. Frustration quickly mounted and even if you didn't return the gadget to get something else you didn't recommend it to others.
Once a product has been released following the 90/10 rules, the rest of the functionality can be added (or perfected) later. The key is grabbing the customer's loyalty through a great OOBE, and then adding functionality with tasks that aren't done as regularly as the 10 percent in the rule. Sure, critics will sound off about the lack of a given functionality, but it's the buyer you're trying to win over, and you will by following my rule.
Apple is not the only company making gadgets that gets it right, but they almost always do it properly. That's why I use them as an example of how to do it right. You can argue about Apple and its products if you're not a fan, but hundreds of millions of customers understand the value of getting the 90/10 rule right. Over and over again.