What is really different about AppSource is that you can find and load an app instantly. It’s extremely similar to using an app store on a smartphone or tablet. You search and find something you’d like to try, click a button, and it deploys directly to your application.
The traditional process of contacting an ISV, getting your partner to merge the code, and managing the deployments across multiple environments is going away. Software providers who develop apps for AppSource use an extensions model that adheres to specific rules that allow you to install/remove those apps at will.
Because of the new deployment model, AppSource is not backward compatible to older versions of Dynamics AX, Dynamics NAV, and Dynamics CRM. Though even if you are on an older version of those applications, you may still benefit from AppSource today based on Office, Power BI, or other applications. Additionally, you can still look through AppSource to see apps that may be worthwhile and part of upgrade planning.
(Note: Some Solution Apps utilize “Partner-led Trials,” even with those you are provided a cloud environment to try out the product and put in contact with the partner to demonstrate various functionality. Supporting the trial deployments in Azure is part of the listing requirements for the Partner.)