Here's what I know.
The most common issue I see is no URL and keyword mapping. You only need 1 URL to rank a keyword. Content should be mapped out first. Nearly 80 percent of our new clients have duplicate content, titles, descriptions. Always having to clean up content.
HTTPS can improve your rankings and traffic.
Social influence can greatly improve a page's rank. But when the social signals die, the page will fall.
Relativity is more important than ever. I've seen domain names (exact match keyword domains) have great ranking power when they link back. Even when the content isn't exact of that domain, the URL influence has been heavy for SEO.
Greatest improvements in SEO are when these exact match domains link to you.
User metrics are important. Crawling rates are important. Engagement on a page can help the keywords rank.
I still see un-friendly mobile domains ranking well, not as big as a factor for SEO as some people say. Speed is important.
I know some are old, but in my case studies, these are some of the most important I've seen as of late.