Firstly: please note this is not my pass phrase.
The whole sign up form is a superb example of both interaction design and microcopy. Firstly they use password unmasking so customers can see what they're typing in and don't have to waste their time typing out their password twice. Interestingly although the placeholder text in the input field prompts for a "4 or 5 word phrase only you'll remember" the grey hint that appears under the field as you start typing also gives you the option of "a short complex set of characters". There's a little room for improvement in that text as it starts with "Remember…" but they hadn't previously said I could use a more common password-type affair. Ultimately, though, Simple is, er, simply giving their customers the ability to create whatever kind of pass authentication works best for them; they just care that it's strong enough. It certainly beats my current bank which doesn't even allow special characters in its password (don't get me started on that).
The use of "grading" for the strength of the customer's password is sensible as it's a concept everyone will be aware of from school. Another positive is the microcopy that displays none of the awful "error, password does not validate security strength [but we won't tell you what that is]" crap that so many other businesses infest their forms with. It's interesting (to me at least) that although a C grade is the minimum required by Simple, they class it as minimally secure. I'd be interested to see if that text encourages people to go on to make their password more secure. As Simple is still in "friends and family invites only" mode at the moment, I'm guessing they don't have the data for that yet.
My one regret when playing with this form was not checking to see what code there is for accessibility. I hope Simple is being cool and adding in ARIA compatibility. I wish I'd checked.