I'm the founder of WP App Store and would love to hear first impressions, suggestions, etc. from the HN community. Happy to answer any questions as well.
Yes, I did discuss this with my advisors and we did consider rebranding to avoid potential legal issues. In the end, we decided against it, mainly because others are using the term "App Store", like Amazon and Shopify. Last I checked, Amazon was winning their battle with Apple as well.
Yes, we're exploring the idea of allowing free themes and plugins in the store. It would allow easy upgrades to PRO versions. I also like the idea of making donations easier for free theme/plugin developers. So instead of "Buy & Install", it could be "Donate & Install" where the user would have to explicitly skip donation before installing the theme or plugin. All of these are just ideas at this stage though, no plans yet.