2 big issues.
First off, the entering of email addresses and remember me stuff is confusing for my customers. We sign up people for a free trial and take their credit card details before we sign them up as users. Even quite technical people have dropped out of the flow after signing in with stripe thinking "I've given them my email" and so people haven't properly finished the signup process because of this (I'm guessing I can probably get this email, however I'd still need to prompt them for a password).
The second big issue is that the constant changing of the form kept breaking various integration/acceptance tests that I had written. This was pretty frustrating as it seemed that I would get a different box from time to time and my tests would start failing.
I get the desire to A/B test, and the desire to build a network of users who have already given their credit card details (obviously amazing for mobile) but it would be nice for us customers if we had a flag where we could switch it off.