1. The letter flipping, as another commenter mentioned.
2. How the keyboard colors the keys after your guesses.
3. The social sharing, with the simple (text only!) sharing of your scores.
4. The "once-per-day" nature of it.
5. It's all basically in a single static JS file! I don't have to wait for anything to load, don't need to sign up or login, yada yada (of course, I pretty much expect the NYT to fuck that up, but oh well, that always happens over time).
I could basically see a ton of different iterations of this that wouldn't have been any where nearly as successful.
