> Should all requirements be written as User Stories?
Sure, if that is what the team is comfortable with. Like Joshua said, Scrum does not prescribe here. User Stories are one of many Scrum Technique Patterns for representing Product Backlog Items.
In terms of whether to assign story points/velocity to defects and technical debt -- Scrum is also largely silent on that topic. There is no one best way to handle that, and the optimum way depends quite a large amount on team and organizational context.
Here are some of my thoughts on that topic: http://www.scrumcrazy.com/bugs http://scrumcrazy.wordpress.com/201...s-to-bugs/ http://www.scrumcrazy.com/Scrum+Str...nt+Backlog
^^ See the bullet under "considerations" that starts with "Resolving technical debt"