The Japanese are very organized and welcoming to tourists. Great embarkation and help at ports.
The Diamond Princess has several features that I like: a Promenade deck that goes all the way around the ship (you have to climb and descend stairs at the bow), a terraced pool at the back of the ship (you can take buffet food out there), an observation deck on top of the bridge.
Food is generally pretty good, wait staff work hard to serve you. Kai sushi was very good, Sabatini's slightly disappointing.
Adequate, barely. Besides the bed, just a single chair at the vanity. The dreaded shower curtain. Bring your own conditioner. Aforementioned saggy beds. The cabin doors have a (necessary on ships) strong door closing mechanism, resulting is lots of noise/vibration when neighbors use theirs. Excellent cabin steward - Malcom.
Yokohama has a cool cruise port. Interesting town.
Take the time to go to Kyoto, one hour by car, 20 minutes by bullet train.