The pause between typing a word and reading the next one doesn't feel good. I think the game would flow a lot better if you could see the current word and the next word. Then there wouldn't be that jarring pause of trying to read it as quickly as possible.
The challenges were pretty easy (I usually type ~120wpm), except for the Lorem Ipsum one. I figured out that it's a lot easier if I change the font. It might be fun to show people their wpm, and maybe show the wpm requirement on the Challenges screen?