Is Rigor The Right Word?

In recent years education has focused on the 4 R's.  Rigor, Relevance, Reflection and Relationships.  I can't disagree with Relevance, Reflection or Relationships.  My issue arises with Rigor.  In reading the definitions of Rigor I can't find anything that would be appealing to a teacher or student as it pertains to learning.  Words like harsh inflexibility, a condition that makes life difficult or uncomfortable, strict precision wouldn't make me want to come to class.  I think we might want to use the word engaging.  When we look at its definition it seems like this is what we are aiming for.  We can have challenging curriculum if we make it engaging rather that rigorous.  When I talk to students they don't mind being challenged if teachers make the subject interesting.  I would appreciate your thoughts on the subject.