What I appreciate about so-called flat design is that's low-key by nature, and by not calling attention to itself (no bezels, few gradients, border colors, etc.) it can - when used correctly - get out of the way and let the user focus on content. E.g. https://dispatch.io

An example of the opposite is so-called good design, where skilled designers set out to make a visual statement and, in the process, obfuscate the content. E.g. this site, where every article is more than half-buried by a ginormous image: http://www.fastcodesign.com