After 4 Royal Caribbean cruises we decided to give Princess a try. We were on the Caribbean Princess April 5-12, 2009 and had the same impression for a majority of the staff (but not all).
I have a theory why.......gratuities are added automatically and there is not a user friendly process to add more.
With Royal Caribbean.....when your grat is added automatically they still give you an envelope you can present to staff.
On Princess I felt like the staff knew they had your gratuity.....knew they weren't getting any extra.....and behaved accordingly. Disappointing.