Ok, I'll play!
- Hole in the handle is too small. Make the handle bigger on the inside to get two fingers in.
- Cross-section of the handle is too round - puts a point load on your finger. Flatten the surface a bit to spread the load.
- Put a sharper corner on the outside top corner of the handle so you can use your thumb to counter the rotation of the cup on your fingers.
- Put temperature sensitive marking on the outside so you can tell what temperature it is at a glance. Think Coors' "Blue Mountains".
- Rim is too thick. Thin is better for sipping.
- Put some funny graphics on it. Funny is always good.
I'm sure there'll be some really good ones!