Taking into consideration the person's background, I mainly look at the candidate's ability to code and solve design problems. While having knowledge of the various stacks out there is cool, they are not the best judge of one's ability based on the interviews I've conducted over the past few years.
Working for a large corporation will often limit the options available to someone who is coming from a contracting or freelance position. And no two companies are ever alike when it comes to process and development.
With that being said, I focus on the candidate's foundation. I want to know how long they have been coding? What their favorite projects were? What do they love and hate about Front-end Development? I start with the basics and challenge what they think they know about Front-end Development. I want to know what their real world experience is. Far too often I've come across many who know how to use libraries and all the latest buzz words but can't explain the differences between IE 11 and Chrome. My favorites are the self proclaimed JS experts who struggle with writing a IIFE.
Also, In my world, a Front-end Developer who doesn't know or understand how to use Photoshop will not go far. Mainly because it's a a tool we used just about everyday. My boss thought it was silly of me to ask such basic questions until a contractor he hired uploaded a 8 meg image on the home page. Knowing how to work with the creative assets provided to bring something to life is key. It helps with collaborations between Creative and Development and makes for a well rounded individual.
My team is made up of various individuals with skill levels ranging from Junior to Beast-mode. The one thing they all have in common is a strong foundation and the desire to learn and push the edge to produce quality products.