@onimatrix: More Suggestions - I think you can remove steam from the options of elements for creation. Also... is it possible for electricity to make gas glow instead of ignite? And if that's possible, perhaps the glow from electrified gas could affect plant life differently - perhaps mushrooms grow instead?
I have been in love with this app ever since I started playing (circa v4.5) and it just gets more and more amazing. Some ideas: slow burning building substance (like wood, perhaps?); a way for hanging icicles to form (I'm not sure how, perhaps ice becomes "sticky" instead of loose?); a cement-like substance (powder that hardens once water touches it). Wind element (I'm not sure how the user could control it, maybe there are four winds, one for each direction?); what if the user could set the gravity (power & direction)? And finally, I'd like to see stone as an alternative building material (that is still affected by elements like stone, but does not succumb to gravity). Even if you add nothing more to this app, it is awesome.
Glitch! Well not a glitch, but a design flaw. You can click on the box to cancel a purchase - but when I am rotating a soldier, sometimes I forget about that fact and accidentally get rid of the soldier - which ends up making me lose money (or worse - I can't buy the soldier back until the next wave). I recommend an "are you sure you want to delete this guy?" prompt or make that option not available as you rotate your soldiers.
The luck required to click objects in the fraction of a second they are on the screen (during the bomb levels like 28) takes away the enjoyment of solving them.
"Doc, am I going to be okay? Is it a case of 'Group Germs'?"
"Well, I'm afraid it's much worse than that. You're intestines have been completely reduced to striped diagonal ramps, surrounded by solid tumors. IF, and that would a BIG IF we can strategically use your immune system AND we are able to beat these germs, then we will have to do a intestine transplant right away. "
It's a good start. The controls work pretty well. I caught myself holding my breath a couple times as I nearly avoided crashing into the rocks below. I look forward to see what you do with this.