You will be prompted for your Atlassian account details. You only have to login once to complete your registration.
No thanks.
Be warned, paying users upgrading from 2.x to current 2.3.x will be forced to sign up.
To the earlier comment, we have gotten a lot of feedback in regards to the sign-up to Atlassian Account. The fact is that you always required a license, so you had to sign-up anyway. But we took the license away and streamlined the sign-up into the product (yes, there are issues with people behind firewalls that we will have to address).
Personally, I don't think requiring an account is asking too much for a free product and down the road it will help us to provide a more seamless integration for people who use other Atlassian tools as well.
Cheers, Jens