Diana's E ability, Moonfall, can interrupt channeling ultis, like Katarina's, Nunu's, Karthus's (when his not dead), Malzahar's, Shen's, Twisted Fate's and/or even prevent Kayle from getting away with her ulti. Her R ability, Lunar Rush, should be better cast after her Q, Crescent Strike, because, this way, you will be able to cast it twice before it gets on cooldown.