I would add a leaderboard, and then from the second word on, as you keep guessing something like "better than 10% of users", to keep me motivated.
I also wonder if you add different languages, for me it wasn't that easy in English.
As implemented: There is a clear "game over". You did as well as you could; try again tomorrow. That's great. The length/difficulty gets harder as you go on, so getting to 18 is a true challenge. Also great. The letters are in the order they are in, no reshuffling. This is also great. You can't just spam reshuffle until you see it; you have to make do with what you got.