I really like this, it kind of reminds me of "Hangman"!
A few ideas, if you don't mind:
1. Have a "normal" mode and "master" mode, where the normal mode has words with maybe 5 letters and the master mode uses 7 letters
2. Have an archive of previous games - kinda sucks that I can't play the old ones!
3. Contrast on the up/down arrows on letters isn't clear enough, at least not for me (colour blind)
Thanks! Archive is in progress. As for the length of the words, I initially though a 5-6-7/esy-medium-hard, but It seemed very easy to guess the 5 letters once you play some games. I'll reconsider.