The old icons had a functional aesthetic, the new ones don't. Really the design is flawed for the new icons on so many levels.
I mean if you're going to make them simple go for solid colors with high contrast to the background like the helmet (base), plane (airport), and castle (fort). Those are no frills, entirely functional. They are easy to see, easy to identify, and easy to differentiate from each other. The only problem here is the lack of borders... since things can get bunched up on the map you need borders. The military unit icons are the best examples.
If you want detail, those building icons for factory/capital/city/town/county are on the right track. They are easy to see, easy to identify, and easy to differentiate from each other.
Your system resizes to 16x16 for everything except the Capital. Don't make the icons 24x24.
Fire whoever decided it was a good idea to lower the contrast with the background and add gradience. FIRE THEM NOW! THEY DON'T KNOW WHAT THEY'RE DOING!
That's my opinion.
While I'm on the subject of icons... who decided it was a good idea to make the old defensive icons green and offensive icons pink? Defensive products use red icons and offensive products use green icons! Maintain some consistency!