"A senior developer is expected to be fluent in client-side and server-side contexts, and know his or her way around Python, Ruby, or whatever other arcane tech

"A senior developer is expected to be fluent in client-side and server-side contexts, and know his or her way around Python, Ruby, or whatever other arcane technology requires taming." Being fluent in client-side and server-side contexts are a perk, otherwise I'm doing the same thing every day.