Improvements to the data system make it easier to debug the propagation of data across elements. Simpler array handling is featured along with batched data changes, which can improve performance and correctness.
Polymer 2.0 removes the need to use Polymer.dom for DOM manipulation, making it easier to use Polymer components with other web development libraries and frameworks. In addition, shady DOM code has been repackaged into a reusable polyfill, instead of being integrated into Polymer.
For migrating from Polymer 1.0, Polymer 2.0 features a backward compatibility layer that allows elements written with the 1.0 API to target the 2.0 API and native browser features. A hybrid pattern allows developers to port elements to a common subset of features in Polymer 1.0 and Polymer 2.0. Those elements can run in applications targeting either version of the library, according to Google. Note that some minor template changes may also be needed for migration.
Polymer 2.0 also ships with an improved polyfill loader that loads only the necessary polyfills for an individual browser, resulting in a reduction in payload. Google's goal behind Polymer is to help developers build Progressive Web Apps, which are focused on providing a more native-like application experience, working with multiple form factors and browsers.