I know they are all one class ships. Except for the QE2, but the cabin grade you go in will depend on what you get in your cabin.
Most major cruise line have a class system based on cabin grade just like cunard(QE2,QM2,QV not just the QE2) they just don't promote it as such.
The main difference with Cunard is your assigned dinning, the other benifits vary between cruiselines but include things like
Special events
In room waiter dining
Full room service menu
Priority bookings in alternative restaurants.
priority bookings on tours
Butlers
Concierge
Private decks and lounge space.
Inclusive drinks
portable(ship wide dect) phones
DVD/CD etc
Personalised stationary
Daily flowers
Canapes
..........etc.
We also never use a travel agent or book through brochure, much cheaper ways than that
TA are nearly always the cheapest it is just finding the right one that will refund commissions or one that has block bookings.
Sometimes direct with the cruiseline can be the same price/better since there are no discounts available.
(trade insiders can obviouly get better rates but they are exclusive so don't count)
Would be very surprised if Princess, competing in same market, did not do likewise.
Princess in the basic cabins have sacchets for shampoo and lotions and a bars of hard soap still a lot of ships have gone over to gell products in dispensers. ( quality makes little difference as anyone coming from a hard water area will notice a biger difference just from using the ships water.