An easy answer is Color. Many of the colors available are bright and vibrant. This is great for toys and trinkets, but not much else. As prototyping and replacement parts become a bigger portion of what is printed on consumer 3D printers, more natural colors will be needed to more closely match household and marketable colors. Variations of Grey, Brown and tans would be helpful for my personal prototyping needs.

I currently need to spray paint parts to get the correct color. Although this looks good, it takes much longer and covers the "3D printed look" that some people get excited about.

As a second point... carbon fiber/carbon nanotube filament, or anything with high tensile strength and fracture resistance. I trust these materials will develop fine with all the attention they already have

-Brian