Hmm. Hard to put my finger on it...
I just can't like this game. After about the first five levels or so, I was thinking "Um... why exactly am I playing this? I'm not really enjoying it, and it's not getting any better."
I felt the same thing about the earlier ("harder") version, though it took a little longer that time.
Level design? Collision detection? Game control? Visual interest? Beats me what's wrong with it. It's just not much fun.
In the interest of fairness I'll reveal that I tend not to like platformers a whole lot, and that's basically what this is.
Oh, one factor I can definitely point to, and it IS a level/game design problem: because you restart instantly when you die, on some levels this means that you're going to immediately die again because you were probably moving right or left at the time. That gets annoying real fast.