Fun. The sudden death feature is interesting. It forces you to both type as fast as you can while making absolutely no mistakes. That's actually probably a very good habit to enforce.
Small bug: if you switch to the "daily" tab, the bottom of the dialog is cut off. This also prevents you from clicking "Play again!" without switching back to another tab.
Also, I think you should consider both allowing the space bar and return to complete words. Pressing space between words is much more natural. Not typing spaces is fairly unnatural typing.
Alternately, later levels could be short phrases or sentences including spaces.
No idea why you need a signup... You don't even ask for an email address. You should try and remove this requirement.
Anyway, after running through the game, I really like it. It's good fun. I'd probably say that the round was about 10 seconds longer than I wanted it to be though. UI is nice. It's pretty fast, so didn't notice any problems.
Gave it a shot. It's well-done, but there should really be an enticing reason for the visitor to sign up. If I get a link from someone and the first action I have to take is to sign up for something, 99% of the time I close it right away, unless they've personally given me some compelling reason to try it.
Nice, very smooth and fun. If you're going to require signup, perhaps at least integrate some way to share/tweet scores and view the leader board without having to go through a round of gameplay first.
Small bug: if you switch to the "daily" tab, the bottom of the dialog is cut off. This also prevents you from clicking "Play again!" without switching back to another tab.