hey there, glad to see you posting here at pixelation

this doesn't look to shabby, mate. It certainly is odd seeing pixel art from you rather than CG/3D. I can see where you're going with this simplistic style, so I'll try not to mess with that much. Heres a quick critique for you. I didn't stay true to the "pixel purist" ways, but most of my critique is compositional...

things to note:
- all foreground / collidable elements have higher saturation and contrast. this helps the player instantly know what he is and isn't allowed to do - and what pieces that involves. For this particular style, black outlines (like I have done here) might be a good direction to take.
- no pure grays. Grey can be a useful tool if done correctly, but for the most part, you probably don't want to have any large areas remain gray. -especially for a game. It's dull and isn't interesting for the player. even on the scoreboard I added a hint of blue to the highlights.
- adding depth. In addition to adding saturation and contrast to the collide-ables, adding depth will give it the extra "pop" that the game needs. I've done this here by extending your drop-shadow. it's one idea (and probably the simplest), but certainly not the only one. Changing the perspective, or showing the sides of walls might also be good solutions.
- the hud: it should be the most prominent feature the player sees, seconded only by the game sprites themselves. Make this pop out with higher contrast and saturation as mentioned before.
- use complimentary colors. no color is ugly itself, but there certainly are ugly combinations of colors. Be sure to use colors that work with each other to make a unified scene.
take care,
-Dan