In the test instance, when I click "Subscribe," there's a popup notification with some text, but it goes away very quickly. This is frustrating if you want to absorb the information but can't read quickly enough. Any way to have it stay?
Tested in http://newsletter-test.wmflabs.org/wiki/Special:Newsletters
I agree that the time for that floating dialog could be longer, or as Pete say just make it stay until it is clicked.