Personally, I feel like a confirmation for any form field is insulting. Is it there to prevent typos? I can almost understand it for the password as it's obfuscated by dots (hence harder to see if you made a mistake), but I can "feel" when typos happen, and if one happens when entering my password for the first time I'll just clear the field and type it again.
In the case that I accidentally register with a typo-ed password, I'll just reset it through my email (which I never mistype via. Typinator). Alternatively, the password probably got saved by Chrome.
If anything, I'd remove the Username field and just make the email address the username (assuming the backend could gracefully handle the transition). My chances of registering go up exponentially the smaller the form gets, which - I would postulate - is the primary reason the "log in with facebook/github/open id" buttons are so effective.
Looks great. It would be awesome if you could just enter a URL to start discussions with.
As for paying accounts - maybe keep 3-5 free design feedback free per user. That way people can try it, get some value from it, and the professionals who want to keep using more (and wanting to store old feedback) will want to pay.
If for each box, you could somehow select all of the scripts and styles inside, and allow them to be edited, with the edits being visible to all in the app, then this tool would not only be great for quick discussions about design, but also a way to quickly agree on it's solution. Don't forget being able to somehow copy that code somewhere for when you actually make the change!
A screenshot of the change with the dialog could also allow freelancers to keep a record of approval with evidence!
While this may be a lot of work, the benefits could mean this app being used extensively in the design and UI process. Let me know if there's something similar integrated.
EDIT: This type of functionality is something that I would pay at up to 50 a month to have because it would save time opening an editor, making the change, reviewing the change, and finally committing the change when all I want to do is get approval from a client.
Look to the stars with this, it's a brilliant tool, and has many possibilities. Good luck!