With the advent of the fox uprising and my recent dive into the world of planets (hur hur), not to mention the ever constant struggle of dealing with pixeling trees and character hair, I have decided that I'm going to put together a thread showing what I mean when I offer the critique of, "You need to define your forms more, because the texture looks like it's on a flat surface."
People often seem to be able to deal with forms when there is no texture, but as soon as we add the texture to our forms they lose all their previous shape. This is particularly noticeable with pretty much everyone's first pixel tree.
This is because they lose all shape of their forms when we use all the highlights and shadows of our object on each, individual piece of texture (each bang of hair, each bunch of leaves, each piece of fur, and indeed each feature on their planets gets the full range of value/brightness). Instead what we need to do is only use the amount of value available to us in any given region and just hint at texture, rather than draw out every single leaf.
So let's dissect this with a simple example.
Here I have a nice, round, beautifully orange ball (using
Arne's 16 colour palette):

Beautiful. That is a ball and you can see it is a ball. This is a well defined form.
But we wish to add texture to our ball! In this case some fur (it is orange like a fox after all).

We begin by adding some edges between the light and dark colour. Here we heavily imply the shape of fur.
Notice how I am cutting into the dark area with the lighter colour. This
changes our form slightly. It looks like it's being lit slightly differently than our initial ball, because there is now more average light going further around the ball.

We remedy this by adding some darker shade over some of the lighter area.
Again I've used obvious lines where there is fur. I put some shade on the first bits of fur texture we added to give that feeling of depth.
Notice we have
changed our form again! We're striving to keep the
average value of our transition area of light and dark the same as our original form.

We'll just scatter a little more darkness here and there to give more
implied texture. We're not actually defining any more fur, and importantly we have left large, open areas
entirely blank!
It doesn't matter though because on the edges between our light and dark we have heavily implied the existence of fur, so our brains will fill in the large areas with imaginary fur for us*.
More form changing.
Side note*: This is a very important part of pixel art. Hinting at details and letting our viewers' brains fill in the gaps rather than trying to use our very valuable, limited resources to draw every piece of fur (which often causes noise and confusion).For a final step let's just add some fur around the edges for more implication of the textures.

And there you have it. With two simple colours I have created a readable, fur-covered ball.
Attentive readers will notice I have decreased the average value on the brighter side of our furball through all this, so honestly our ball is slightly less bright than it was originally, but we have kept our form and that is the important part.
A bonus byproduct of this process is that we were able to break up the solid, boring line between light and dark on the original ball using texture. This means we can, and should, use textures to add interesting details around edges when possible.
I shall continue to update this as I think of more things to add/change (or come up with better examples) and would like the opinions of other members with regards to the topic and this post.