I really like the Arachne-line-art one (the way you colored/textured it is very different from the rest of the takes I've seen).

As for Mr. Wierd Moster, he seems to be missing something and I don't know what it is.

Best guesses:

Detailed eyes (instead of just white with a black dot).
AA'ed wrinkles?
Use shading instead of plain lines to give shape (especially to the folds around the purple things)
Darker shadows
The spouts the flames emerge from look very flat, and the flames themselves look like blood-soaked straw brooms. ^_^
I don't think the orange light from the flames plays into the rest of the picture enough.

All in all I would focus on:

Making the eyes and tentacles look more organically connected to the body (Right now they have a "drawn as an afterthought" look to them, even though you probably didn't draw them as an afterthought).
Giving him some more volume (darker shadows would help at least partially here, I think).
Flames (I know how hard fire is. I don't even try anymore =P But there are a number of posts here involving fire and I am sure you can find something to help you along).

Continue your very strange but good work.