I am not writing these reviews to be mean, only to help, so please read the whole thing before exiting and assuming I'm just another jerk who likes calling your animations bad.
Before talking about the actual content, do you expect anybody to read the text on the loading screen? The font you chose and how poorly it was written would suggest not.
The animation starts with a really poorly drawn stickman who doesn't even have an actual face. The facial expression was so badly done that I can't even tell what you were trying to pull off. He's looking to the right and yet his mouth suggests a smirk. It doesn't hurt to actually use circles or pupils for eyes, or to spend the extra two seconds to draw an actual nose.
Your background is empty. Like, literally, empty. It's a window, a desk and a really badly-drawn chair, but I'll elaborate on the chair part later. It's just plain bland. When you're watching a cartoon, look at the background. Even if there isn't that much stuff in there, there's a perspective and at least some sense of balance.
I understand you don't have a tablet and are forced to draw with your mouse. But why do you think the line tool was invented? Smooth and curve the lines to your liking and you should have some nice line art. The brush tool is a superb choice, but only if you have the right equipment.
Windows provide an outlook of something, not just blue oblivion. At least white linear scribbles on it would be better than nothing, as it suggests shading of some sort. For the colors you used for the wall and the floor, the minimal colors you used, were dull.
I am not criticizing you for your simplistic characters and design. Look at the Powerpuff Girls - it's very simply drawn, but tiny subtle details distinguish it from looking horrible and disoriented. Looks like your floor is curving and your window is rounded where it shouldn't be! Such poor architecture.
That table literally looks like a cardboard cutout. Like an elementary schooler who cut out construction paper and glued it to the set. There's a such thing as a perspective. I'm not asking you to pull out something of Pixar quality, but look up a 3D square and look up your plain square. Which one looks better and more realistic? The 3D one.
When the guy walks in - it's Night of the Undead Inconsistent Shading all over again! Your shading is all over the place. Where's the light source anyway? Anyway, let's move on to the art of the guy.
His head is a circle, and not even a circle, it looks like he was born with a birth defect or something. The eye was too big and too sharply drawn to look like he was genuinely angry. Look at an angry guy and look at his face. This isn't even a vague representation of what you were trying to pull off.
The body isn't even a body, just some square thing you draw as a body, but it's not. This can also be solved by looking at anybody's body and I can tell you nobody's body looks like that. You're probably reading that last part and saying, "I'm not trying to draw a self-portrait here, I'm trying to draw a cartoon!"
Cartoons have rules too, and if not obeyed, would result in a horribly disoriented drawing/animation. Looks like his leg is thinner than the other!
Shape tweens ARE NOT your friend. Inbetweening is. Manually drawing it and manually timing it is always going to look much better than a crappy shape tween. It's obvious when someone tries to cheap out of it by using a shape tween and shape tweens are almost never acceptable. The timing was good though, despite the actual drawings.
What I'm about to say here isn't related to the cartoon, but is also very useful. Don't take criticism to heart. I'm not saying not to mind it, but even if one of your reviewers are being mean, they may raise valid points. Even if someone leaves poorly articulated criticism such as "Art was bad, animation was bad. Didn't make sense," the fact something upset them should make you want to change something.
Hopefully this helped, since this took twenty minutes to write and I used up all of my space. Good luck.