Hi Steve,
you wrote:
What would you say are the top three factors that make it look unprofessional?
Not that I'm an expert either, especially not in Inkscape, just some suggestions in general when creating vector graphics based on real objects:
* Use a high resolution photo, and put it in the background. That helps you get the proportions right. Instead of a photo, you may use a technical drawing or even a rendering you created with a 3D program.
* Trace by hand, and try to limit the number of control points. That gives smoother curves. In your picture, curves are uneven, some look like drawn freehand (factor 1).
* Use the color picker to get colors right. Use transitions, and try to simplify, working with gradients and transparency. Use the same level of detail throughout.
In your picture colors don't match (factor 2) and level of detail is inconsistent (factor 3). For example, the side windows is flat blue, while the front window has a gradient but one which doesn't match. The side mirror looks nice and vivid, with a reflection.
HTH
- Felix