I like your demo. Nice atmosphere, I like jetpack as aditional equipment. I think that it shouldn't be in inventory, but activated as option. I like how screens change instantly when you're jumping off the screen.
Secret areas are good for this kind of games but I didn't see what was that thing that I've collected and what is for. Stages with time limit are interesting but don't use them too often, they can be tiring.
Spider like creatures are very unoriginal, I bet that you can come up with something much more interesting.
I agree with PixelPiledriver
If I'm holding 5 blocks, it might be nice just to press '1' 5 times to drop all of them.
Also controls are a bit all over the place. I'm running from lava using arrows, pressing Z to jump, but at the same time I need to go up and press a number to drop the block without a mistake.
Game reminds me to "Zombotron" game which is good.
Overall good demo.