Security. Also 'growth hacking'; I'm not sure if that will actually up your salary, but knowing "everything" about SEO, analytics, a/b testing, email marketing,

Security. Also 'growth hacking'; I'm not sure if that will actually up your salary, but knowing "everything" about SEO, analytics, a/b testing, email marketing, blah blah can definitely specialize you within an org. EDIT: Employers also hire for 'tech stack' specialization, even though it's usually not necessary. For example, I'm only really recruited because I have demonstrate-able Rails expertise. In reality, I've also worked in Node.js and Java environments, and I'm pretty much just as productive after a few weeks.