From an interviewee's perspective, these are my favorite interviews too. I get to talk about something that interests me, and have a discussion about the different decisions I made. If the interviewer wants, they can dig into specific details, or ask more pointed questions about the programming language. I find trivia questions off-putting, and tend to limit the depth of discussion into my skills.

And to the people saying that this necessitates projects outside of work - I don't see why it does. You can talk about school projects, projects from previous jobs, etc.