If you holding a sword you can't break anything in creative mode. Only if you not holding a sword you can break stuff.
Linked issues
Comments 2
140+ Duplicate of MC-18368, please use the search function to see if your bug has already been submitted. Currently over 58% of tickets are being resolved as duplicate.
This is intended. It was added in 1.6, I believe. It is so that killing mobs in creative does not cause you to accidentally destroy something.