Anyone scared of the new W3C custom components?
Introduction to Web Components
I think the custom components and shadow dom scares me most. Current XPath and CSS selectors don't support shadow DOMs.
Imagine developers can create their own tags for elements that your automation tool doesn't support. This has been problematic in the Native application world, but for a long time on the web apps, developers were only limited by what was available in HTML. In the future they can create their own element types.
The shadow DOM i think is particularly scary. The idea is to make content easy to swap out quickly. However current CSS and xpath selectors weren't designed with shadow dom in mind.