I like most of the Civ V innovations: 1UPT, city states, better religion, social policies, unique abilities for each civilisation. It makes the game much more repeat playable.
I dislike diplomacy. Constantly having to check whether Civs have enough money for RAs, to buy resources etc and then having to contact several civs to find the best deal is tedious. Surely there could have been a screen showing each Civ's best offers? Plus the complete mystery of the diplomacy. I assume that supplanting a civ as a City State's ally is not as damaging as taking their capital, but who knows?