Amazing systems knowledge. My previous senior was the chief architect, and a life long C++ developer. His systems knowledge was amazing, unlike all the people i knew previously who were either Java or PHP or Ruby coders. Dynamic language folks have almost no knowledge of the system their software works on which makes them script kiddies at best. Experience isnt enough to be a senior, your knowledge domain should also expand with time. Which for may doesnt and they are still developing in Ruby/Rails or PHP or Java. They havent tried writing or learning any other language. But the C++ guy since had learnt maybe 3-4 new languages. Including elm, js , Haskell etc. I feel that Knowing the latest Ruby/Rails API isnt enough to qualify as an architect because it changes every yr. That isnt an indicative of how good a programmer you are or how you think. I had .NET and Java language colleagues earlier who i had discussions like why are you starting two docker containers when one is enough? And i had a hard time explaining them the purpose of docker or process isolation. I feel a good systems knowledge is crucial to developing software including webapps. The managers with good systems could identify problems quickly while with others i had discussions like lets just deploy 1 docker container to what is docker etc..