A jack of all trades is a master of none, they say, but they are wrong. You don't have to master it all, but you should be comfortable throughout the webstack.
Good understanding of the basic building blocks of fullstack development. NoSQL/SQL databases, HTTP(S), REST/SOAP, Ajax, HTML/CSS.
A backend technology of your choice(Python + some framework, node.js, Go, Scala, Java EE, RoR - whatever rocks your boat)
As far as architecture goes, you should understand how to best serve the mobile clients in addition to the web front
A frontend FOTM technology of your choice - but most of all the ability to adapt to new frontend gimmicks.
Great communication skills and a sense of humour (optional)