I don't see why people got stuck so early in this game - I made it to level 22 before I had any problems at all. The issue here is just that the game is not motivating at all. Everything from the graphics, sound, and gameplay all scream generic to the point where I can't understand why anyone would bother making this. Maybe you created a nice functional platform engine and decided to release it on its own? I don't know.
I stopped trying for stars at around level 8. I quit at level 22 because I had to make a jump between a floating crate and a robot in midair and I kept dying in midair even though it was plainly obvious I hadn't collided with anything at all. Boo.
Glitch on level 6. Level 8 is too hard in my opinion even though I like challenges it seems impossible. Please change the level. Then people can enjoy the game instead of raging. (Some people might)
Sooo, I clicked on the first level and the guy teleported up to the top left corner of the game screen, right behind the stars. I've refreshed twice. He does not want to come down.
EDIT: Changed rating after glitch was fixed. Solid platformer but the jump is not awesome, the space bar switch between does not always work and level 8 sucks it.
Level 8 isn't all that bad once you figure how the junk compactor really works, but you still need a keen eye to find the safe spot before it crushes you. I agree with the comments posted below, this is a sudden difficulty spike and should be switched with one of the last levels.
As for myself, I got stuck on level 16 for a while, since it requires good reflexes to control the falling boxes, but finally managed to beat it. Now I can't wait to see what's next ;-)
By the way, I had a small problem with Newgrounds medals: the standard on-screen message is never displayed and the small icons under the main window light up only after refreshing the browser. I think this may be discouraging for some players, especially for those who finally managed to finish level 8 after so many failed attempts.
To sum it all up, Just Button is a good game with variety of challenges and not too frustrating, if it weren't for the problems with difficulty curve, I'd give it full 4 stars.