I believe that what is wrong with the sentence is that it never says what he was the cofounder of - the order of the words in the sentence is a little funky. It should be rewritten like this: Mick Fleetwood is best known as the cofounder of the rock band Fleetwood Mac, where he played the drums.
I think either excited or confused. Since the first sentence is well, with an exclamation point, I guess I would lean towards excited. It would help to know more of the story.
