The reason for optimism with Weis is a combination of the top two factors.
First, I think most have recognized that Davie was a strong strategist, particularly on defense, but was apparently a poor motivator. He could tell you the theory of every play, but couldn't make it work regularly enough to win. (There are other reasons to dislike Davie though, the Moore lawsuit, etc.). Davie seldom got blown-out.
Willingham was a good motivator when he needed to be, but was inconsistent and didn't know crap about gameday preparation. Willingham had never been a coordinator and was not even a huge success as a positions coach. Willingham often got blown-out.
Weis certainly knows the strategy, but has hired assistants that have also proven themselves as top position coaches. Still, it remains to be seen if Weis is the total package. I think blind enthusiasm now is a bit misguided. On the other hand, it does seem like EVERYTHING has gone right since ND hired him: Dream staff, recruiting surge, and a fourth superbowl victory.