For me, the combat redesign is what will make or break a new civ.
Hexes and new graphics aren't enough (and ooh, ranged bombardment, which is modded back into Civ4 anyway).
They have to make the combat system deeper, and figure out a way to remove the incentives for stacks of doom while still keeping combat simple and elegant.
I think they thought that collateral damage in civ4 would be enough to prevent SoDs, but it wasn't. I hope they don't think that ranged bombardment collateral damage will be enough on its own to prevent stacks of doom for Civ5.
I'm not optimistic though, its a challenging design problem. But that's what Civ *should* be doing.
The economy design works pretty well. The combat design, not so much.