I've really enjoyed this thread. We've sailed on QM2, QE and QV and are on QA next month. Then we are trying Regatta in October so it will be interesting to compare.
I've only been in Britannia cabins, inside - balcony but have never felt the class distinction others mention. I'm aware of the Grills cabins but then all ships hve different grades with perks for the dearer ones.
I especially love the library (though I have to stop myself reorganising the shelves!) on Cunard ships and the lectures and quizzes. We're looking forward to a casual vibe on Oceania. Our Regatta cruise is a fly cruise, so we can travel lighter whereas QA is from Southampton so we can take all our posh gear!