What are you looking for in a developer?
What characteristics do you want?
What core concepts should they know?
What would you want in a portfolio?
Platform related: What do you want in an iOS Dev and a Ruby Dev?
Platform related: What do you want in an iOS Dev and a Ruby Dev?
Remember, as a developer, everyone who asks you for "help" is your customer. Be it a CIO or your cube-mate.
If I am interviewing you, I want to know how YOU will solve my problem. Tell me about a time when you helped a customer solve a problem. What the problem was and how you gathered the requirements even when the customer didn't know what they wanted. Tell me how you helped a customer think about different what-if scenarios.
By you telling me a few stories like that, it will allow me to dive deeper into areas both soft and hard skills.
Having gee-wiz things in your portfolio is good but if that doesn't help the customer solve their problem, what good was it. Tell me how your gee-wiz things solved a customer's problem.
The interview process equally checks technical viability and culture fit, and the interviewee will not be hired if they don't pass both with flying colors.
To answer your questions: The characteristics I look for depend on the culture I'm building within the team. The core concepts are more or less "Do you know how to solve this problem?" not necessarily "Here, solve this problem in this interview, without any of the resources you would normally have." A portfolio isn't required, but gives us another window into your technical ability. A GitHub profile chalk full of open source contributions, or a StackOverflow profile with high reputation showing very high communication skills are always a plus.
I don't have any experience hiring iOS developers or Ruby developers, so I'll abstain from answering that question.
Hope this helps!