Hi Kuki, the real answer depends on what you are comparing Cunard against, which level of stateroom you would book as it is still a 2-tier class liner, and which of the Cunard fleet you are looking at.
The builds of QE & QV are the same only with different decor, and follow an "out of the box" industry standard design for large cruise ships, whilst QM2 is completely different.
As Henry43 said, they still have the white glove afternoon tea service, which unlike some lines (Fred.Olsen) is included in the price of your voyage.
To us, the QM2 feels more formal than her younger sisters, but that may be because she spends so much time running the TransAtlantic route, so has different objectives. For us QV (and hopefully QE) are far more in line with other 'British' brands, where formality is still required, but the entertainment & venues on board feel more relaxed.
If you let us know what aspects you are wanting to compare, and which liner you are thinking of trying. Then we will try to offer a more precise reply.
If you take a look at the website Cruise deals holidays, worldwide, comprehensive directory, impartial-advice
and follow the drop downs, you will find general descriptions, as well as info on staterooms, leisure, entertainment, restaurant, dress code, size, rating, etc.
Alan & Katrina