DISCLAIMER: I apologize in advance, I am being brutal here. It will seem rude, and it is a bit, but you asked and I want to answer honestly. I sincerely hope that what I say doesn't offend anyone, and I mean no disparagement to any of the many artists whose work this game includes. I love art, I even love ugly art, and I love this game, too.
The textures are far too bright, the animations are hilariously bad, the GUI is spread out all over the place, nothing is scaled right, but most importantly colors and shapes are thrown together with absolutely no hint of coherence. There's no separation between foreground, background, and sprites. The important things are given exactly the same visual treatment as the unimportant things; background objects are exactly as crisp and vibrant as enemies if not more so. It looks, frankly, like a bunch of art assets stuck together completely at random.
Other games' art styles are largely irrelevant, but for an example of a similar aesthetic done competently, take a look at And Yet It Moves. For an example of an extremely colorful game that's still pleasant to view, try Botanicula. Those are both low-fi indie games with a gorgeous artistic style, and if you want I can give you a list of all the others I have installed to prove my street cred or whatever. Hell, most of the time I prefer indie games to big budget titles. The problem with the graphics isn't a retro style or a colorful presentation, those are fine things, in fact they're often good things. The problem is that the game does those things very badly. I love the game, but it's ugly, and not even the good kind of ugly, like Picasso, or Minecraft, or punk rock. The ugliness doesn't effect my enjoyment beyond occasional eye strain, but it's unfortunate, and I'm kind of amazed that anyone would dispute it.
Luckily it looks like we'll be getting official texture pack support soon, and I suspect people like me will be considerably less noisy once we can easily replace the art. I mean, I've never even mentioned the music since I replaced that. Wait, crap.