One of my foremost thoughts is that, while yes having roughly 20% of web traffic being mobile is definitely something to take note of, that still leaves 80% of the traffic being non-mobile. With this in mind, while I do see the benefits for "mobile first" designs, they must be made with care. In the end, desktop users are still the vast majority of visitors, so if a mobile first design impacts their experience then it's hardly a viable model.

Also, while I'm also not a huge fan of one-page sites, I think that there's a lot of utility in incorporating the long-form elements into multi-page sites - especially the homepage. While still having the full-content inner pages available, a long-form homepage gives some often crucial control over the order and flow of the content that the user sees when first visiting the site.