Anticlimax Boss: The Emperor of Blobolonia. All the player has to do is throw an apple jellybean at the Blob in order to knock down a jar of vitamins, which instantly kill the Emperor. He doesn't even attack the duo.
Ear Worm: That theme which plays throughout the majority of the game.
Good Bad Bugs: The infamous coconut trick which allows to remove lots of enemies. There is even a glitch where you can get a glimpse of a final boss and remove all the enemies that way.
Most challenge levels involving the Cola Bubble or the Root Beer Rocket tend to be this (except for the final rocket level, which is surprisingly long and brutal.) The penultimate challenge level is also pretty easy too, consisting of more rampaging with Blob's mech form.
That One Boss: The Beast at the end of World 2. It's a pain in the patookie for two main reasons: 1) The Beast is fast, agile, and adroit. The Boy is not. 2) No matter what he gets attacked with, the Boy dies in one hit, and if he does, then you have to start the whole fight over again from scratch. And you can't quit mid-fight and pick up there, either—if you want to stop, you have to go through the whole level over again. In an article for Gamasutra, the developers themselves admitted that they didn't think this boss through enough. Oops.
Uncanny Valley: Blob manages to look cute no matter what form he takes, until he copies the Boy's appearance with his Mint Double form and ends up looking kind of unsettling.