Honestly... I loved it. It wasn't really difficult, but I thought it was on purpose... I think it would have be great to add a scenario to your game, you know. The universe is very interesting, so why not? I'd love to know what happened on Earth, why does this research lab exists, where do all those parasite-plant monsters come from... But in the end, it was a very decent demo. I love art pixel art, its one of my favorites kinds of 2D art, and I know how compicated that must have been to draw all those sprites.
For the remaining, I've read a lot of reviews below saying the game was laggy, but I didn't lag (on Chrome).The only thing that was frustrating for me was about the weapons, the upgrading system and the fact you lose your gun when you die... I lost the flamethrower I found by pure hazard, so I'm a little bit.... Argh...
In the end, that game has a real potential. I'm totally impatient to see it when you'll finalize it :)
PS: Sorry for my english if I have made any mistakes, I'm trying to do better but I know I'm not perfect ;) Keep up the good work !