+1 to learning Plug before Phoenix, too. Sometimes I feel like I have god mode activated when co-workers or friends are hung up on the connection life cycle or not sure how a piece of Phoenix works/scales. I'd add a basic understanding of OTP to that list as well.

Phoenix does do an excellent job of getting people to try Elixir, kind of like how Rails did for Ruby. Phoenix has also added a cool abstraction in Presence/CRDTs. Now that people don't have to implement that themselves I wonder if that will be as useful to people like how ORMs stormed onto the scenes in the early 2000s.