I do a lot of commissions, so this is from experience.
1. My biggest problem is the client comes without thorough research. They have no idea which material to design for or never even read the material descriptions.
2. No dimensions, just a pic to go from. Research is not the best use of my time and can really drive the price up.
3. Clients often are very slow to answer emails through the design process. I am always waiting for someone to get back on a design question.
4. Clients wanting to know up front what it will cost to have printed before we even start. Often I rough out a similar size shell to get a rough idea, but I never see compensation for that.
5. People need to read and understand the Shapeways process and time-lines including shipping.
6. People should look at the designers work and feedback AND read read the descriptions.
7. Pick a designer that draws the type of item you have in mind. There are sculptors, train, vehicle jewelry and mechanical designers. Choosing a jewelry designer to make a mechanical part and vice versa does not work.
8. Just because it's small does not mean it's takes less time than drawing it for a particular scale.
9. Don't have a designer draw for detail and cheap out and try to have it printed in White material.
10. Keep in mind the Designer and Shapeways are two separate services.
11. RESEARCH< RESEARCH and MORE RESEARCH material ready when approaching a designer. You will get a more accurate idea of cost entailed and address issues early on.
Hope this helps