So you have to file an App development application with PayPal to use this plugin?

The is what the plugin asks for, I don't think it is possible for regular plugin users.

I logged in PayPal developer portal and checked out the application process, it is extremely long and sophisticated with requirements for tons of information, supporting documents and understanding of App development process and technology. And you guys expect every user to jump through such hoops just to enable PayPal payment gateway in this plugin? Seems quite strenuous.

Why is an "Application ID" even needed? Why isn't the API credentials in my PayPal account enough?

You must register this application with PayPal using your business account login to get an Application ID that will work with your API credentials. App will be using "Adaptive Payments -> Preapprovals" for charging user after the goal is completed. Register then submit your application while logged in to the developer portal. Note that you do not need an Application ID for testing in sandbox mode. Please remember not to use same sandbox account for funding and reciving founds. More Information »