Well that was really enjoyable - it was a near ideal contribution to the puzzle platformer genre.
I loved how each puzzle was contained on a single screen so you never had to worry about stuff you couldn't see yet having an effect on anything. I also loved the increasingly growing number of features you had at your disposal to get through them - very nice pacing and difficulty curving.
There are only two things wrong with this game, in my opinion, and one of them is just a minor nitpick: I'd've rather have been able to jump with the up arrow and use space to pick up/drop the boxes. As for the other thing: that guy seemed to take way too long to stop, especially when going at full speed - a much higher coefficient of friction would have made the game handle a *lot* better.
There was also one aesthetic note I wanted to offer regarding the 3-high pits present in a lot of levels that indicate you've failed the level if you fall into them. One of them had a series of platforms on its side that allowed you to jump back up to the start of the level while all the others forced a hard reset of the level despite having room for a similar feature. If they all had that feature (or even if they all didn't), you'd probably see a slight uptick in the overall opinion people have.