One thing that consistently annoys me is the design of paint tube caps. Most manufacturers -- even the best ones -- deliver their tubes with caps that have hard edges and digging little channels. The effect is like running your fingers over a rasping file. I use a fairly large palette, so after setting the 40th color out, the insides of my thumb and forefinger really hurt. If I do it often enough in a week, they are so irritated that it looks like I have a skin condition.
I recently tried a tube of Daler Rowney paint for the first time. Setting it out was heaven, since they've obviously thought about this. Smooth, gentle, rounded edges... twisted off like butter... soooo nice.
Paint manufacturers, take note! Do your human factors research and give us a more usable product.