In my experience, talent and passion are the keys to effective teams.  It's not better defined roles.  In PM Tip#14: Great teams have members that defy roles, Brad Abrams writes that the best teams defy traditional roles and responsibilities, and I agree.

Role-Focus
When role issues show up, it's usually a sign there's other problems.  Here's common problems:

• You're missing a critical role.  In sports it's more obvious.  You wouldn't start a baseball game without a pitcher.  On a project team, where the work isn't well understood, it might not be obvious you're missing an important role, but symptoms show up over time.
• The open work isn't obvious.  If the team isn't sure what to do next or who's doing what, it leads to problems.
• There's too much stepping on toes.  There's probably just not enough teaming or communication.
• The work is unevenly distributed
• Somebody is out of their comfort zone.
• There's a mismatch in talent for the task.
• There's a mismatch in passion for the task. 
• There's a lack of clarity around what good looks like.  You might think you did your job, but somebody else thinks you only did half of it.
• There's a mismatch in values.  If you want to knock the ball out of the park, while somebody else wants to just punch the clock, you'll have role issues.

Carving out roles to help orient the team is one thing (who's on first, who's on second ... etc.)  Carving out roles to act as contracts is another.  I'd rather work on teams where the focus is on collaboration and results over contract negotiation. 

Results-Focus
Here's a few keys that help with a results focus:

• Build a shared vision of the end in mind.  If the team can't see the finish line, there's no way they can race there.  
• Know the work to be done.  Granted this is a continuously unfolding process.  At the same time, you don't want to go off to Antartica without the right skills on board.  If you know the work to be done, you can get the right people on the team.  If you know the work to be done, people are generally more willing to sign up.    
• Match talent to the task.
• Match passion to the task.  Passion is the key to productivity.
• Stay adaptable.  Adaptable teams survive and thrive, where rigid teams fail.
• Pair up in complimentary ways.  For example, pair a dev with a tester to swap perspectives.  Pair a strong thinker with a strong doer.  Pairing up is a great way to keep momemtum and share learnings.  It's also a way to help avoid stepping on toes   

At the end of the day, I push for results-focused teams over role-focused teams, but it's probably because I'm a fan of getting results.