ChaIR lead animator Scott Stoddard gave a talk on the first day of GDC2014 detailing various aspects of Infinity Blade's design that helped make it such a successful property. He touched on three key elements: core combat, metagame, and analytics.
Stoddard pointed to the game's focus on having players use simple actions -- swipes of the finger -- to elicit dramatic feedback in the game. Substantial effort was paid to making things feel dynamic, malleable, and fun. "Good gameplay happens when you balance interesting play abilities with interesting play challenges," Stoddard said, pointing to the game's parry system. That said, the team needed to design a simple, but deep metagame overlay to give the game's many interesting abilities meaning.
ChaIR wanted to make every item in the game both valuable and meaningful while circumventing oftentimes convoluted traditional RPG mechanics. This is why they decided to put the leveling system in the weapons instead of the character itself. This metagame mechanic, Stoddard says, feeds right into the game's step-based difficulty mechanic, which eschews a traditional "ramping" approach to offer players a more dramatic sense of accomplishment when they finally topple one of the series' many God Kings.
Stoddard also detailed the team's devotion to collecting and reacting to analytics. He related a story where, despite the team's love of the mechanic, data showed that players were only pulling off the original parry move 10% of the time -- which, in their eyes, meant it was very broken. They responded to the data, making the move more complicated by integrating a tiered success system; as Stoddard says, binary is often detrimental to a game's dynamic feel. An analog system makes the game more accessible for more players but allows a mechanic to retain its challenge for more skilled players.