I love this picture and if you don't mind I am going to compare it to your mech warrior as it features a lot of the things I talked about in that critique.
The subject has really good detail and a neat perspective, and it stands out significantly. This is the kind of detail I was talking about (moderately) in the mech picture, and you should try to build up on this kind of stuff. Even in this picture it could use some more work, but it's still significantly better than the previous.
The environment is great, especially the sky. The sky and horizon are really build up with neat clouds and the colour contrasts with the ground. It is somewhat messy, but it's not particularly a bad thing in this picture. Try painting a bunch of different sky scenes, and get the feel for how clouds are constructed. They aren't terribly difficult and can be done very accurately very quickly if you learn what to do.
Wow! This is REALLY a work of art! I mean, you can see all the details clearly, and (although my screen is so small, and I need top keep scrolling up -_-) I can see it all right there. The painting technique is excellent, it really looks real! As if you had painted it on canvas The colours are perfect for the scene. The ground, the sky and the machine all seem to fit together. And if there is anything that I think could be slightly altered, maybe the lines (around the machine and protruding rock to it's left) should be slightly dimmed. I know you made them that way as to let us see all the lines and details, but the way it is now, they look rather strange, like a light. But that is just my "over-seeing" eye xD It isn't really major The overall effect of the image is extremely compelling to see more! Well done!
It's apparent that you really tried on this one, I think it turned out rather well actually, but I do see a lot of stuff that could be improved. For one, things seem monochromatic - by that, I mean you need to have a wider range of color, saturation and hue to make everything look more 3D and realistic. Right now you have browns and greens - but it's a desert with sand, so I'd also like to see some reds and oranges, maybe even some purple for the shadows. Also, you still have lines left over on the ship; the point to speedpainting is not to have lineart, so I would avoid those in the future for this technique - using them as guidelines is good, but don't leave them behind. I like the texture you gave things, it looks like you made a few custom brushes or at least used different ones, which is good - though it seems a bit repetitive in areas (like the ground), and looks like you used the clone/stamp tool or something. I'm not the master on texture (I barely use it myself, honestly) but I'd check out what he did for his piece to make it look more like sand instead of blotches. Your perspective is good, your shading is good - your blending needs a bit of work I think, but overall I think you did well! I'd try another one, see how it goes! :)