What I think you need is more definition, less colours, more control, sharper edges and all that.
I took one of your tanks and gave it a quick makeover.
At first i set down some very simple shapes as guides and then i added some edge highlighting and smoothed things out.
Then more detail, some dirt, whatnot. I based this style somewhat on the style of the Raiden Fighters arcade game series, which uses similar rendering (with more colours to make it smoother)
The big benefit when working with pixelart is that you can pretty quickly recolour stuff with new ramps and such.
In this folder you will find some sprites from some Raiden Fighters games.
http://ptoing.net/post/rf/If you have specific questions just ask
