There were so many problems with this special trip, that its hard to know where to start. Lots of other passengers felt exactly the same way. We have no intention on travelling with Cunard again, unless they do something to make amends.
Firstly the big one - being lied to by Cunard, and then this being spun as a "unique opportunity". The ship was faulty, and had been arranged to go into dry dock long before we even boarded, but they withheld telling us until the day after sailing (Christmas Eve no less). This was to make sure the ship was ready for the World Cruise, which made us feel really special. This meant the trip was never going to Lisbon as advertised, and our time in Madeira was curtailed. On the previous cruise to ours, the ship had visited Cadiz for a "wet docking" during which they had failed to repair the faulty azipod propellor. This was causing unpleasant vibration in the Britannia Restaurant on 2-deck, and across a number of cabins. We measured it as 80db. We didn't enjoy the dry docking at all - it was noisy, it was hot in our cabin at night, it wasn't luxurious at all to be on the pool deck aft being loomed over by giant yellow cranes. We don't believe the ship was actually fully repaired, as people are still complaining about the vibration now.
The food was consistently disappointing. Tasteless and uninspiring. Nothing we ate was of a standard that we'd pay for in a restaurant. Fine dining it was not. A recurring theme was things that looked or sounding nice, just being boring.
Midships 5 deck cabin was generally fine, although we encountered cleaning issues when boarding (old food left on the balcony, seriously stained net curtains and dusty surfaces). The balcony had rusty areas and the door handle had corroded badly. The TV reception was frequently awful, even when the ship was stationary. Temperature was fine, except when we dry docked when it was too hot because the AC didn't work.