There is no one particular component of your running shoes that is the “most important” part: It is a sum of the parts that matters and the sum is different for each of us. Ultimately, it means that each of us is different and different shoes benefit each of us in different ways.
There are some components–the pieces of the shoe–that are important to look for, no matter what your needs are.
The complete ground contact midsole (like in the superduper New Balance 1400 v2, pictured here) is a midsole/outsole combination that is in touch with the surface of the ground at all points. It is flush with the ground, and there are no midfoot trusses or bridges to separate parts of the midsole. This feature is becoming more and more prevalent on the shoe walls, and it is one of the most overlooked and underrated features in running footwear.
Complete ground contact effectively makes the heel to toe transition smoother. The smoother the transition, the more comfortable the ride. When you’re running on complete ground contact, there will be less of a lever effect , making the platform more stable underfoot. It’s a win-win for you. The more comfortable the shoe, the more in will benefit you and the more you’ll enjoy running, longer.