Forget these haters, and for the love of god don't add "achievements and unlockables" as that one guy suggested. I see kinda where you're going here and think it's really cool direction.
Here's my criticism: It seems like there is currently no "right" or "wrong". Personally I would like to get some feedback on whether or not I'm hitting the buttons at the right time. Even if you dont think there is a "right" time, I assumed that I was supposed to be keeping the lyrics in sync with the singer.
Second, if I ever fell really badly behind, I had to mash the space button to get my lyrics back up to speed with where the song was, that was kind of odd and jarring.
Also, it wasn't clear to me how many times I was going to have to press SPACE for one of the long words, like "diamonds" in Rihannas song. I kept pressing it the wrong number of times. Maybe add some kind of visual indication of how many times the person is going to have to press SPACE for the word.
But overall I enjoyed it and think it could be really cool.