This kind of things are perfectly fine if you're aware of what kind of hit it might have on the overall visuals of a game for example. Let's say you have hand made not-so-perfect character animations and then this 3D saucer guy. The perfection of the volume in the 3D saucer would really stand out, no? So you gotta make sure you use some trickery to make the aesthetic consistent, like Mathias pointed out.

Yes, this makes a lot of sense. This piece, however, is a stand-alone and not part of anything in particular, so I guess it's not an issue here. But it is an interesting caveat to consider in applying this technique elsewhere.

To me, graphics are only necessary to the level they are enough to convey the basic abstract meaning to the game mechanics, but for that they must be used properly. It's all about matching multiple contexts together.

For example, a mere triangle in Asteroids was enough for a spaceship, but only because the game responds to the user commands by moving it like a spaceship. If it moved like a car, the triangle would suddenly be considered a car. A square can be a bullet if it behaves like a bullet, but it could also be the free roaming hero, like in Atari's Adventure. Audio comes in handy here giving us a familiar contexts (explosions, bangs, crashes, splashes), but even the audio doesn't need to be very accurate.

But this is just game mechanics, in my opinion, which is rather bare in any meaning or purpose. Gameplay, at least to me, is the reason behind those chosen mechanics. If you just want to create a metaphor for life (like in Passage), a exploration-friendly fantasy world (Sword and Sworcery) or an action-adventure platformer, the mechanics will just be picked to convey that message. All the extra graphics, sounds, music and storylines are just there to deepen that purpose, to give it some extra meaning and maybe some other message. Gameplay is just the motivation for interaction, and "art games" try explore just that.

So what defines a good game to someone depends more of what level of concrete-ness they expect and understand from that certain gameplay, and if any of the features are too abstract or lacking, they'll feel like they're missing something. If you just can't imagine the square being your hero, then you won't feel motivated to be the square.

This lacks "power" just like my other ones did. PLEASE, I need advice on better ways to get that effect. Be specific, and I don't want anybody to just edit it in for me because I can't learn like that.

That "power" effect comes from the timing of the movment. Notice how your character moves forward in a constant speed? Your reference sprite's animation looks better because there's a very subtle "charging" of the movement at the top of the jump, before the kick, which gives the illusion of weight and force.

This is one of the 12 basic principles of animation (anticipation). It's key for dramatic actions.

Interesting question. I don't think it really matters unless the dithered boundaries actually meet.