this is an interesting topic as it is something we have been wondering about also
we have done 3 trips with Princess and have another 2 booked, so over all we dont find the staff poor, but at times not helpful
our first trip Syd/ Akl on the Diamond was wonderful. Predominantly older Americans were the passenger make up. We did the Akl/syd return and found the crew not that helpful anymore and put it down to the fact the novelty had worn off. But on the last few days of the cruise we got to chat to staff that were getting off in Sydney and found it wasnt our imaginations - it was the fact that the passengers were mostly Australia/Kiwi and tipping isnt their style. This seriously impacted the service given. We started to tip and wow how things changed!!. A major example of this is the soda cards. We couldnt get service at all in a lounge and were barely tolerated when presenting to a bar.
We have just come back from 17 days on the Sun Princess, and found it to be poor again. This time we tipped as we went along, but the dinner waiters were poor, they made constant mistakes on the meals, then when we tipped they showed us better service on a table of 6, which became embarrasing!
the soda card was a waste of time as we couldnt get service at all this time, and so now wonder why the company offer them when their staff wont honour them. Our room steward resented our presence, after he had met the cabin occupants and then were changed to ourselves (another stuff up with the staff - they seperated mum, dad and 2 kids by 2 floors by offering one an upgrade!) It took a week for him to accept we WERE the entitled occupants, but once more it took a tip for that to happen.
We are back on the Diamond again, so it will be interesting if the service is the same - or if we notice it is a race thing. Australians and New Zealanders do not tip. It isnt in our customs and the crew know it and treat us accordingly I think