I'm not sure whether QI is more entertaining when its informative, or when the comedians get bored of Stephen and band together to keep it from being informative.
This is why John Hodgman didn't fare too well. It also didn't help that he was a "fifth guest" and the usual witty-and-cunning Brit comedians wouldn't let him get a word in.
I noticed Rich Hall uses his nationality as a comedic foil when it can be riffed on, plus he's a stand-up and improv veteran. Hodgman's exaggerated New England Intellectual character doesn't quite work in QI context. He's best when condescending and icily passive-aggresive, declaring intellectual superiority with smarmy hostility. So he seemed quietly polite and really out of his element amongst the all the ribbing and wisecracks.