I really like the first image in the thread as a sketch, but the rest don't really do it for me. Part of it is that I like the original palette more, but that one is a bit contrast-lacking, seeing it next to the other one. The others lost some of the personality with the design change and also look really unfinished and sketchy, but AFAIK aren't just sketches. If they are and you're going to refine it at some point in the future already, ignore the rest of this post. But otherwise, I think the issue is that you have a lot of horizontal lines at the ends of things and areas where you have some 2px jumps. Basically it looks like the whole thing was made with a 2px brush. Adding some more 1-pixel wide lines in some places (especially the tentacles) would make it look a lot smoother. Also look at Arachne's edit for a great example of how to make transitions between areas of light and dark less jaggy.