Non-spoiler thoughts of Ultra Moon(compared to Moon)

Pros:

1. UI and menuing is much cleaner, snazzier, and better.
2. New interaction between you and Rotom Dex make it more than just a talking Pokedex.
3. Story changes at the start are mostly very nice, and actually give a reason for a specific thing you did in Moon that never really got explained.
4. Music choice is better.

Cons:

1. Lazy cutscene importing. A detail in the story was changed, but the cutscene that accompanies it does not change, thus making the cutscene not match the change.
2. Related to #1, a story change was for the worse; it lowered the impact of why you did what you did.