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One quality that most casters have is an ability to talk about the game at a high level. Communication is a huge part of working as a team and Cora has always been great at that.
Another part is understanding the meta and why it changes. Which cards are most important to each deck, and how a deck might change if a card was added or removed.
Another important aspect of final design is understanding which cards are fun or exciting. Casters and streamers tend to have a very good idea of which cards make for fun gameplay and scenarios.
Cora did great on all of these aspects and we are lucky to have her as part of the Hearthstone team. I look forward to working with her.