I'll speak to point 5.
Civ4 unconditionally rewarded expansion of the empire. There was essentially no victory condition where an increased number of cities wouldn't be beneficial:
1.) Conquest/Domination: Military production is increased significantly with more cities.
2.) Space Race: The moment a city generated more commerce than it cost in maintenance, it was a net addition to tech speed.
3.) Diplomatic: Increased tech speed gives you the technologies/gold you need to bribe Civs to be friendly.
4.) Cultural: Allows you to build more temples and increased tech speed gets you to key techs faster.
5.) Time: More cities results in a higher score.
Mastering Civ4 was a matter of mastering expansion of your empire. Virtually every situation rewarded you for expansion of your empire. It wasn't a matter of if you should expand, but when. This is not the case for Civ5, and I consider that to be a benefit. It encourages varying gameplay styles.
Small empires are not at an inherent disadvantage to larger empires. In some cases (as in the case of cultural victory), you are rewarded for maintaining a smaller empire. In other cases, you can still maintain some parity with a larger empire (largely due to the global happiness mechanic, separation of research from commerce, and city-states). And in some cases, you are at a disadvantage, such as military conquest.
This makes gameplay less one dimensional as it pertains to victory conditions. It does force you to make difficult choices (e.g. should I expand to that 4th/5th city for a military/tech advantage and effectively eliminate cultural victory as an option). It does attach more permanence to your decisions than the decisions made in Civ4. It means the cost of your decisions won't be relegated to solely opportunity cost. But I consider these difficult decisions the basis of a good strategy game. I consider the long-term ramifications of your decisions an improvement. The flexibility you miss, I am happy to see gone. It rewards well-thought out plans and foresight. And to me that is what a strategy game is about.
The game certainly needs massaging. But ultimately I feel once more work is put into balancing the game, the decisions are going to be harder to make in Civ5 than they are in Civ4. The path to victory will be less evident than it was in Civ4. It's not there yet, but I look forward to it. The game design is certainly better set up for it than Civ4 ever was.
So big empires aren't (or can't be) culturally accomplished? I think the Romans and others would have something to say about that. Seriously though, I have to agree with the OP that the scaling for cultural victory is unbalanced.