And implemented them in much much better way than cIV did. Example: Religion, Espionage, World Congress resolutions, city flip... and the list goes on.
This.
Civ V devoted whole expansions to themes Civ IV had as add-ons, so be fair. And if it hadn't been for Civ IV and the way it introduced the concepts, it's unlikely Firaxis would have thought to make the UN, and maybe even religion, a major game theme.
Having said that, I wouldn't say city-flipping is handled better - I miss tile-flipping. And although in concept Civ V's World Congress resolutions are better-designed and more varied, the AI seems to be quite some way behind Civ IV's in understanding how to use them.
I've preferred Civ V to Civ IV for some time, but I played Civ IV just before BNW hit for comparison. In some ways that highlighted a lot of the ways in which BNW changes feel "Civ IVish", particularly the more exploration-based early game and more varied viable tech paths and early building choices (and, less positively, the more passive and seemingly less individualistic AIs). However, Civ IV feels more polished and 'finished' than BNW - patches may change it, but while Civ V is now in essence a more complex game than Civ IV, AI weaknesses that seem more pronounced than in G&K, balance issues, and new concepts that seem great but underdeveloped make it feel less complete than G&K:
- Ideology's great, but I'm coming to dislike the predictability it gives end-game diplomacy and the fact that the end-game is rarely balanced - you have one culturally dominant civ that prompts everyone else to adopt its ideology, and only a couple of civs bucking the trend. I haven't yet seen a near-equal pairing of ideologies, Cold War-style, neither of which is obviously dominant. I'm starting to pine for a post-ideology Information Age that tones down the effects. It makes all prior and other forms of diplomacy almost irrelevant, up to and including dropping nukes on people's favourite city-states.
- The fact that on large maps with default settings you're going to run out of named artworks etc. says it all about how well thought-through tourism was. It's got novelty value for the theme bonuses, artifacts and watching the culture battle between civs unfold, but it turns out to be very limited and there's more interest to be had in the varied ways of preventing Wonderspammers from foisting their ideologies onto you than with actually playing the tourism game, which is very linear and lacking in diversity.