I just pushed a new version with a few significant changes in response to your awesome feedback. You can play it in the browser, or download via the links below.

Changes (possible spoilers):

• Reworked spiders. They were previously very annoying to deal with because of how long it took to dispatch each individual one. The new spiders are, I think, just as difficult, but they're a lot more interesting and fun to play around and can be dispatched quicker. Watch out!
• Added a visual smoke particle when using the jetpack, as well as a new feedback sound effect when you try to use the jetpack but have no energy. This is because players were saying the jetpack was sometimes not working and it felt like a bug - this should help explain that it is supposed to not be working.
• Replaced the white ghosts that shoot lasers with a mechanical turret, which makes a lot more sense. Now unopened chests will be represented as broken/disabled turrets, rather than simply disappearing altogether, which should help players understand the key interaction with opening chests.
• You are now awarded a gem for every heart that you have remaining when you "give up". This gives the player a small reward for switching worlds on their own terms rather than being killed.
• Added a few new tips at the start to more clearly spell out some game mechanics.
• Added "chests" stat at the end-game screen (number of chests opened).
• Minor: On the stats screen, do not count time spent while paused.
  
• Tech: Use "borderless" fullscreen to enable screen capture.

Thanks to everyone who played the game so far!

Matt