pkmays' comment is spot on and it was inspiring enough for me to spend a fair amount of time editing one of your pieces. My original goal was to make a quick edit that'd clean the sprite up a bit (color conservation, AAing, consistent lightsource) but, I decided to do a major edit because, it'd allow me to give you a whole slew of guidance versus your typical "needs more dither". Heh.
I picked that guy because it was either him or that black wall man that showed the most potential. Personally, I don't find monotone half-man half-rectangles that interesting. Anyways, the linework on this guy was pretty solid so I did very little editing on the actual line art. The mouth, the feather, and the mane were the only things truly problematic. The mouth was decent, had character but, not quite enough. I'll be touching on that later. As for the mane and feather; they both looked really constrained and unnatural. For the feather I tried to give it a more natural feel by shaping it to flow. Your feather's angles prompt thoughts of a pipe or steel wing. The mane doesn't seem to have any rhyme or reason to the shape of it. The segements are randomly sized, shaped, and angled. The positioning of the segments give the impression of some force blowing them back but, the lack of any definition beyond the outline and the randomly sized gaps between the segments makes it just seem like the mane is out of perspective. In my edit I pretty much just evened out the segements; width, length, the space apart, and the angle. I was going for fluffiness and didn't even realize at the time that the mane was blown back so, I kind of failed on both levels there. I guess he's got some sort of harlequin mane.
The next issue was the colors. I quickly made a 12 color ramp to simplify the editing of this sprite. You had too many colors, too close in value, for the piece's own good. There's nothing wrong with having a relatively high color count but, if you're going to up the colors make sure to make them count. I would suggest starting out with a smaller amount of more contrasting colors and adding new ones only as you need them as buffer shades for the first few. 6-8 is probably a good number to start with; make sure that your darkest and lightest colors are part of that ramp. If you're making a piece with multiple color ramps (your blue alien on a green pedestal in a purple cloak being a good example) you should make three seperate ramps, 4-6 colors each, and work from there. I can't really speak because, lately, I've been unable to use more than 10-14 colors per piece but, that could very well be because I make good use of what I have. Either that or I'm lazy. Ha!
Moving on, slightly color related, is the shading of the piece. One of the biggest issues it has is that it looks really really flat. This holds true for every single piece you've put up for display except for the rectangle man. Once you get the line art done on a piece; define it's shading and lightsource with three colors. Those three being the brightest, darkest, and middle-most colors. From there you can use the remaining colors to buffer out the, most likely, overly contrasted colors. It's not important to have smooth lines or absolute shapes at this stage, this is the blocking stage. The blocking stage's purpose is to ensure that your overall shape is not deteriorated when you start adding in random textures, shapes, and shadows. Believe me, when I say, that this can happen very very often. I don't know how many trees I've scrapped because the leaves portray a much different shape than I had intended. Also, important to note, you can/should use your line art as a guide when blocking your piece out but, block out the lines as you make use of them. As you can see in my edit, the mane and where the arms meet the body are shaped entirely by the other shapes around them- not lines.
Onwards and Upwards! Next up comes the commonly used crits- the ones referring to the necessity of dithering and to a lesser extent AAing. The purpose of these are to smooth out the image and/or add texture in the case of dithering. AAing is used for smoothing out heavily contrasted areas in a relatively concise manner. Dithering is used to smooth out large patches of color or to add texture to an image, or both. There is much documentation on both but, I'm not sure if they cover the purpose of them as well as the technique involved with them. Which is why I stated their purpose. Using both techniques only when necessary is a good way to keep a crisp clean pixel art. It's this stage where you start bringing more colors in, if necessary.
Last but not least, you make the piece your own. Anybody can learn how to do all of the above, it becomes very mechanical and innate over time but even if you can manage all of the technichal aspects there is still one thing that seperates you from greatness- and that's character. Something all of your pieces have and something all of them lack. You, most definitely, have the right idea but, you have to push yourself further. As I edited your piece, each area I did instilled me with a ton of insight on how to make it better. The mane, for example, I thought should be bushy and envelop his head. Of course, it didn't turn out that way but, I thought it. I thought it because your mane looked really plain and I sought a way to add interest to it. All it took was one adjective, fluffy, and then I had it; my idea that is. Point is, as you're creating your art, think of adjectives to describe it. It will help you notice bland areas and will help you further define areas that just aren't quite there yet. The mouth on yours looked a bit worried and so I decided to further embellish that in my piece. The end result had little teeth just because my over-exaggeration lended itself so well to them. A lot of good things can come from mistakes or by going over-the-top and the little teeth are proof of that. His eye gave me the impression that he was not a very smart creature and pairing that with the worried look I wondered to myself why was he so worried. I decided that I'd put a butterfly on his nose because it seemed to further emphasize his worrisome stupidity in a childish way. Since he's pretty cute and childish it meshed them all together quite well. It's all random things like this that truly make or break a piece. Practice exaggerating and stretching the limits of what you do and you'll find that your work will be much improved. At first it might seem like it's a bit too much but, it really is one of the most important steps to being a great artist.
So there you have it. You've got everything down but, it's just as pkmays says; the execution is holding them from being great pixel art. Hopefully you can take the knowledge I've given you and up the level of execution in each stage.
