AA = Anti-aliasing, meaning attempts at mitigating the jagginess of outlines. There is a
simple tutorial here, first one on the first page. I did a little of that on the upper outlines of body and head. Sorry, I'm so used to write 'aa' I didn't notice, awful me!
The outer outlines of sprites is usually not aa'ed, because of variable backgrounds. That's the problem with my version of the
antennas antennae.
Banding is what happens when you have 2 or more contiguous identical blocks of pixels. You have some on the lower head outline, I didn't fix it. A very
detailed explanation here.
A color ramp is a subset of colors from your palette, basically the 'same color' going from dark to light. You have 3 ramps: blues, greys, pinks.
The tones in a ramp should be evenly spaced; your former blues had too close darks and a gap after them. Then, the ramps in a piece should integrate, not just stand separately to each other. You shift hues within each ramps to achieve this (and also to make better shading: realistic shading has either bluer shadows - outside scenes with blue sky - or warmer -inside scenes in warm colored rooms). More of this
here and
there.
It's much more than 'rule of thumb' and 'personal pref': making a good palette is an essential element of any good pixel art, and it is by no means obvious. You have to understand how colors work, relative to each other, to the whole, and how the eye works with them. I'm just barely beginning to understand this myself, but it's probably the most fascinating stuff I learnt doing PA.
Search the forums for all these questions, they have been discussed a lot times, here and at Pixellation.
Using my edits: you're supposed to redo them by yourself, not copy-paste them (as it looks like you did, partly). Not that I care the slightest actually, but you'll learn more doing it! ;) Also, edits are usually quick stuff just to make a point, they need to be refined.
Looking forward to more bugz!