Very fun game quite tricky at times.
There were some problems with the game play in general that I noticed. The most annoying issue was the use of the left arrow key for changing levels. In most other balancing, tetris-based games and tetris itself, this would spin the block in the other direction. I used to play tetris so much I had dreams about it and the hitting the left key to rotate thing is so ingrained. I'm sure it's happened to other people as well. You've got it allll stacked up and then you rotate once the wrong direction and you have to start over.
The next problem I felt was that you can not stack items into the gray area at the top. There's a sort of unclear line that separates stackable and unstackable, but limiting the height of the stack seems rather unfair. I consider this to be an open-ended creative thinking game, but when you impose a rule like that where there's a "This is how you're supposed to do it." it starts to subtract from that idea.
Others have mentioned the ads I noticed, but I'll ad this: Three times while playing an ad failed to load and it locked my game. I had to quit and start the game again. Good thing you had an auto-save feature built in, but this is still quite annoying. Perhaps the ads could be timed regardless of whether it loads or something like that.
Other than that, a good and very fun game. I beat all 40 levels of the peaceful mode despite the setbacks but decided to quit after one level of inferno when an ad locked it up again.
thanks for making a good game.