Civ V has a lot more balance of ideas than Civ IV did, with things like excess happiness not being wasted but instead going to golden age, gold that is actually a very useful commodity, more elegant diplo victory, and et cetera.
From a game design perspective, Civ V seems way better than Civ IV to me. Although there are some things that still need work, like a couple of things in the UI still being a little rusty and the overall feeling that you never have enough production to build anything. And then maybe I just need to get used to how slow everything plays. But other than balance and UI stuff (and maybe graphics, I still need to get used to them), I pretty much think every change they made from Civ IV to Civ V was a positive one.
Removal of religions and "first to discover this tech gets this" bonuses, slower culture, increased importance of gold, social policies that make you choose but you don't lose anything you already have when you choose them, removal of cottage spam. These are positive changes in my opinion, so Civ V is poised to be better than Civ IV. And it seems like they took a lot of steps to further balance out the different types of victory, which I think adds strategic depth above and beyond what Civ IV had.