The very first time we're introduced to electrified water, the spark moves so fast that you need split-second precision to get the key. That speed is fine for challenge mode, but when you're introducing an obstacle for the very first time, don't make it that difficult.
Oh, you can indeed exit a level, and return to the last checkpoint you hit. That makes the length of these levels a bit more tolerable, though not by much.
A couple more suggestions (and fair warning, I am not going to stop any time soon; I'm annoying like that). 1) It should be possible to sort achievements by how close you are to completing them. 2) Clicking the Crystals or Money in the top bar should also open the Convert Resources window. 3) Clicking the planet icon in Solar System view should open the Universal Planet Statistics window (and there should also be a dedicated button to that function, because it's easy to get lost, and it's easy to miss the instruction to press "I").
The numbers on buildings which show how full they are are difficult to see. Perhaps replace with a small meter that fills from left to right? Also, instead of having a Collect option, whatever the building has stored up should be collected when you click on it (also opening the upgrade/destroy menu in the process).
Buddha, the clones are your past selves. Clone 1 does exactly what you did on your first run, Clone 2 does exactly what you did on your second run, etc.
 "Idle Blacksmith"){kind=icon}
 "The Wizard"){kind=icon}
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 "Inner Demon"){kind=icon}
 "Vex 3"){kind=icon}
 "oO"){kind=icon}
 "Helixteus"){kind=icon}
 "The Company of Myself"){kind=icon}
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thx, thats come up a few times about the red gem - it was a bit of a lazy choice!