Blackbeard had a look of fear in his eyes just before before and after being punched by Luffy. Didn't see anything like that on him during his fight with Ace. My guess is Luffy had some Haki behind that punch. Then again, Blackbeard could just be shocked that Luffy would dare punch a pirate of his level.

I am very curious about the big event that Blackbeard is planning. Will he take credit for the fall of Impel Down by bringing down Magellan? Somehow, I doubt that is the extent of it.