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As Revvys said, many of the levels feel like you have to do the same thing over and over again.
Beyond that, there are a lot of levels that have little if anything to do with puzzle solving, and more with "HAHA GOTCHA, YOU DIDN'T KNOW THIS LEVEL AHEAD OF TIME SO YOU LOSE". Level 28 is a good example; you barely have time to get the barrel in place to save your fruit, so unless you know ahead of time what needs to be done, you cannot solve the puzzle. That's not good puzzle solving; you shouldn't be required to fail a level (or look at a walkthrough) before you are able to solve it.
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I must have spent most of the afternoon trying to get past level 20 because the timing was incredibly annoying. Strangely, all the other levels I played afterword didn't come close to its difficulty, with the exception of 29.