Some random ideas I have about it (since it's easier to talk about stuff than to build it):
* Your quotes are all from famous books and movies, and you have affiliate links to Amazon... brilliant!
* Do you have a mechanism that minimizes repetition?
* Do you ever need to fake competition?
* Could you visualize mistakes as crashes, to keep things exciting?
* Consider making a keyboard UI for the site (Although I use vimium)
* Build a mechanism to challenge your friends.
* Analyze history and plot it. Take into account difficulty of paragraphs (come up with difficulty rating based on your user data).
* Give typing tips; perhaps it's enough to analyze words that the user mistyped, and offer them more passages with them.
* Improve the CSS. The UI should reflect the incredibly high level of polish in the gameplay.
Maybe I'll have more random ideas when I play it tomorrow. ;)