Something just occurred to me about the rocks - if they were more angular & contrasty they might detract attention from the robot, ie. the focal point... as it is, with the big leap in value taking place in the center, the image does a pretty good job of drawing your eyes to the right places.

.. like that eye. Man I draw skewed. Ffff. Thanks for pointing it out, 7321551. (how would you go about choosing that nick!? srsly)

Random & guaranteed to be available. Call me Jarrad if you like. that's my real name.

There doesn't need to be a consensus. If a crit is correct, it'll make you say to yourself "Oh! Right! That makes sense." May the best crit win.

Well, if you personally like the black lines, keep them. I don't think anyone will be mad at you or anything. There's no point pursuing criteria that you yourself don't see the value of. If you don't assess your work by your own principles you're working blind. (there has been a lot of talk lately about "rules", not sure what's causing it?)

My opinion is that the lines work okay in the larger characters like Castello, who looks like a character from a charming point-&-click adventure, & works poorly with the smaller characters, because, as has been pointed out, the line-weight changes with the relative size of the sprite.

Here's an attempt to convey how thick a 1px line is at this size, perceptually.

If you like thick lines, then that might not be a problem.

If you'd like suggestions for a technique to use, I suggest what blumunkee did up there - light-dependent outlines, I think they're sometimes called. Has lines, easy on the eyes.

I saw a busker playing one on Friday! I thought it was a type of steel pan at the time. He earned my $2. 

Although I think in some instances you can add some visual interest by manually choosing your color... particularly when gray is involved.