The Star Princess along with the Grand and the Golden are considered to be part of Princess' Grand Class of ships.
As is often the case with build alikes - refinements are made as each one is built and then after a few years dry docked and upgraded.
Having spent 49 days on the Grand Princess last year I was quite aware of the ship - it's most glaring fault to me is the lack of a mid-ship stairwell - this meant either a long trot down the hall or using the elevators to get either up to the buffet or down to the rest of the passenger services.
On the Star Princess there is a mid-ship stairwell and I used it most of time I was on the ship.
While the Grand no longer has the Skywalkers lounge - it is now the 0ne Five Lounge - one five referring to it being on the 15th deck - the Star still has the Skywalkers - both were used as the place where the platinum and elite could enjoy there happy hour and treats each evening. The trek to Skywalkers turned me off - did go to the One Five one night on the northbound cruise.
The other difference is down on the 5th deck - while the Grand now has the Alfredo's Pizzeria down on the Piazza and a much smaller Vines - the Star still has the larger Vines and an area where they sell Vines memorabilia which seemed to get little usage.
Those are the only noticeable differences that I can report on from my short time on the Star.
Enjoy and as usual Cheers!