Right now, if you create an account with an underscore, there is a bug where you will not be able to log in. So don't do it right now :)
Are usernames case-sensitive?
Your username will be stored in lower-case, and you should use lower-case when logging in if mixed-case doesn't work.
Where is my avatar coming from?
The forums, bugs and wiki use Gravatar to retrieve your avatar. Head over to gravatar.com to set up an avatar. The avatars on the forum will default to Gravatar, but will also allow you to upload a custom avatar.
I had an old wiki account -- will it still work?
When the new wiki (with the new look) was deployed, old user accounts were not migrated. You will need to create a new account.
I have an old bugs.funtoo.org account, and it still works. How should I proceed?
Create a new Funtoo account with the same username as your existing bugs.funtoo.org account. Then start using your new account to log in to JIRA. JIRA will recognize that you are the same user.