We singled out pop-ups, because it's no secret that they're universally hated. According to research conducted by the Coalition for Better Ads, they're usually what drive people to stop visiting a website or to install ad blockers -- and ad blockers mean loss in revenue. Mountain View will also notify websites if their mobile versions are too cluttered. The coalition's research found that mobile versions typically suffer not just from pop-ups, but also from a high density of ads for such a small screen.
"Replacing annoying ads with more acceptable ones will help ensure all content creators, big and small, can continue to sustain their work with online advertising. This is why we support the Coalition's efforts to develop marketplace guidelines for supporting the Better Ads Standards and will continue working with them on the standards as they evolve."
If the websites Google warns take action and follow the company's advice, then you can expect fewer intrusive pop-ups and more subtle in-line advertisements in the future.