Not "feeling" the audience
Good moderation is an art. And practice makes the expert. So go out there and start your own rooms, invite colleagues, clients, and thought leaders in your field that can attract a great audience. Talk about the subjects you're an expert in and grow your audience. But also be mindful of the people that go up on your stage.
Sometimes people end up sharing deeply emotional stories, and I've been in rooms where the moderator simply didn't seem to understand when a subject was deeply sensitive for the one sharing it. I've seen mods abruptly interrupting people in the middle of sharing sad and emotional stories only to ask for a follow. It felt extremely tone-deaf and the audience wasn't impressed. Many left, others asked to come up to the stage and confronted the moderator. That's not a good way to put yourself in the best light.
Not moderating or setting expectations
Another mistake is not telling speakers and guests what's expected of them, for how long they're supposed to contribute. That can lead to people monopolizing the conversation while others are left unspoken.
A good moderator should let people know when it's their turn to speak, make sure that people don't jump the line and speak ahead of others. Setting up clear expectations is a great way to avoid potentially tense moments and ensuring the conversation flows warmly and orderly.
Not allowing meaningful contributions
Many inexperienced moderators don't have what it takes to make guests feel welcomed and create a mindset that allows sharing and encourages contributions. For example, inviting someone up and asking them point-blank "What's your question" can be intimidating for many.
Moreover, such an approach doesn't allow a guest to contribute with something different than a question. Many may want to contribute with a story, with an experience of their own, and not always with a question. So asking them to ask a question may stifle their contribution. A way to avoid this is to ask them if they a question or want to share something. That open-ended phrasing way people can feel they can speak freely and are encouraged to share anything that's of importance to them. Your audience will feel included and will appreciate you allowing them to be authentic.
Inviting too many people on the stage
Another rookie mistake is having too many people invited on the stage. This will translate into very long waiting times for people to speak and that can lead to frustration.
Another issue with this approach is that when there are too many people up on the stage, it's really hard to see who's speaking and adding value at a moment in time. Participants would need to scroll up and down to identify the speaker and one can end up missing a speaker who was interesting and not know whom to follow.