I really appreciated the focus on listening. This balance of listening and gentle guiding is such an art.

I'd say that in a meeting of more than 3 or 4 people, it's important to have a point of attentional focus like a white board. This gives everyone a place to put their eyes that reminds them what the point of the meeting should be.

Also, sometimes, depending on the content, it's really helpful to repeat back what someone said to clarify it further. Sometimes this happens naturally, but a lot of times people don't feel comfortable jumping in an asking someone questions. It depends on the dynamic. The referee or facilitator is there to help highlight the useful points, and, as you mentioned, make everyone feel heard.

As far as mood, grumpy, stressed-out folks tend to be less creative than people in a good mood. The referee being in a good mood will allow them to listen better -- people tend to be self-focused when they are in a bad mood.The joke thing is a really good idea, but even more effective is to do something that actually gets everyone actively in a good mood, especially if it's a brainstorming meeting. This could be taking a walk as a group, actually doing an improv exercise, or having people take a second to consider something that went well. It may sound a little cheesy, but having an environment where these things are doable (i.e., where people don't feel judged and can freely be themselves) makes for way more innovation. The ideas for this are best coming from the meeting attendees, but, like I said, people need to feel at ease suggesting stuff and they also need to see the value in it.

Lastly, how you present the purpose of the meeting is important. If you say, "this meeting is about fixing the test suite" then people are thinking about how it's broken. Not that this isn't necessary, but you allow people to think a lot more broadly and innovatively if you frame it as "this meeting is about making the test suite more stable." Rather than presenting problems, present possibilities.