I've now done probably close to 100 system design interviews. One of the main things I've looked for in candidates is their ability to identify, communicate, and discuss trade-offs. The next thing on my checklist is their ability to move forward, pick an option, and defend that option. Really nimble candidates will pivot, recognizing when to change approaches because requirements have changed.
The goal here is to see if the candidate understands the domain (generic distributed systems) well enough on their own. For more senior roles I look to make sure they can then communicate that understanding to a team, and then drive consensus around some approach.
