My brain is not artistic and I tend to have to resort to a lot of user polls to help me decide what looks better. I find nothing wrong with the style itself. The sharpness doesn't fit into the default gui I'm using, but I'm going to end up replacing the gui graphics anyways. The main thing with the art style is it has to be relatively consistent throughout the game. I don't know how far exactly this would need to extend. Would the large unit portraits need to be done in a similar style? It's quite the contrast to the production mockup you made. The only thing I have to compare to style wise is the old EFS style and the style of the units I've taken from FreeCiv. I don't really know if any of them is inherently better.
On the note of the production mockup, il-palazzo suggested a pretty good industry method I think I'm going to implement which would work better with tabs.
Back to the symbols...
I asked five people what they thought each symbol meant.
1. 100% correct answer responses
2. 100% correct answer responses
3. 40% correct. Others thinking it was an arrow or wing with a shadow.
4. Got a lot of interesting ones like 'brain juice', but they all had the same theme of biochems.
5. 1 correct answer. 'A shell', 'A door', 'Armor', 'A box'
6. 1 correct answer. 'a button', 'a light', 2 'the letter I'. The guy who got it correct was literally mining in UO at the time and his entire backpack was filled with ingots.
7. 100% correct answers
8. 'A bowling ball', 'The new symbol for lua', 'the gay death star', 'pool ball', 'an easter egg'
I think 3 can easily be solved with splitting the left polygon into two.
5 probably needs more detail.
6 would be a lot more recognizable if it was represented in isometric form and/or with multiple ingots stacked.