How big of a role does differing temperament and energy play in a relationship?

Depends on whether the two people involved have entered the relationship: A) accepting their partner as is, with no expectation nor need for them to change, B) willing to embrace their differences rather than see them as a source of conflict and C) open to the possibility of learning and evolving through exposure to a different outlook on life. 

It can very much play a big role.  I was in such a situation.  I'm in complete agreement with Lyrical's response.  It really depends upon how you feel about the other person, how committed you are to finding a happy balance and making the relationship work.  

I find the biggest mistake people make is in thinking they can change someone.   That's a recipe for disaster.

That depends on each set of individuals who connect.  No two have the same outcome.  Ever.

My wife and I have that sort of relationship. Once she gets going she will go until she is exhausted. I am content to sit and watch her.

depends on the person, everyone is different