To start things off, the art style and animation, while simplistic is very fluid. The facial expressions I especially see myself taking a liking to. The character design of the humanoid character sitting on the chair reminds me of "Yellow Head" from the "LittleBigPlanet" games.
The things I noticed that could use a bit of a change revolve mostly around the voice acting. It sounds a bit flat and bland, and when making a comedy series, good voice acting is one of the key components. For example, the voice acting doesn't match the facial expressions, which can knock the quality of the humor back quite a bit in the long run. What I would recommend to fix this would be spending some time in the "Team Up!" board (this option can be found in the movies tab) and writing something there in search of a voice actor. One thing I do like about the voice acting, would be the clear mic quality. I never missed or misinterpreted any words while watching, which is always a good sign.
Another thing you should do, in the finished product, is have the characters call each other by name, as I'm not sure which one is Zack and which one is Brian.
Also, you have your premise down, but your characters seem a bit like wild cards. Something I would recommend you do to fix this would be to write notes about your characters, outlining their main personality traits.
One thing I can say you have down well would be shading. The yellow square character is one of the best examples of this. It shows you have a good grasp of the colors that you're working with and you know how to use them.
Another thing I like about the short is that despite how the voice acting can cripple the humor a bit, I still found myself liking the ideas behind most of the jokes. The one at the end got a chuckle out of me.
One thing the animation could use would be sound design. For this aspect I would recommend Freesound.org. An example of where sound could be used would be when the humanoid block character's controller broke off camera.
To summarize, this is a pretty neat preview flick. While the voice acting needs work, and the characters could use a little bit more thought, it has fluid animation, a good grasp of it's colors and shading. It has some good ideas and it definitely has potential. I look forward to seeing the finished product.