Anyone who misses this feature can let apple know from the Mail.app, go to the Mail menu and choose the Provide Mail Feedback option.
In case anyone is wondering, the value of the spinning wheel feature is that it showed immediate realtime feedback about the status of each account on an account by account basis which is crucial to users on lower bandwidth connetions and helpful to all users.
The replacments:
1. The Mail Activity pane only shows one bar for all accounts and doesn't give nearly the information or feedback desired, or any breakdown about what is happening with each account. It is pretty much useless but the only real replacment we currently have. (you can activate this using the triangle in the square icon in the bottom left of the status bar in the app)
2. The Mail Status window cannot be left open because it does not minimize with Mail.app and has no way to dock with Mail.app, making it a right pain to use and totally impractical. It can be helpful to diagnose isses, but that is about it. (you can activate this using Window menu in the Mail.app, choose the Activity feature.)
What the spinning wheel indicator did with simplicty and grace in true apple fashion, neither replacment is capible of fufilling with their cludgy partial functionality.
So again, if you miss this feature please leave feedback for the Mail.app team via the Mail.app Provide mail feedback menu option. Thanks.