Religion was a nice idea that wasn't so great in execution. For the most part, each religion acted the exact same as all the others -- it only mattered that you founded one, and that you spread it around so that you could remain on good terms with everyone else. It had too much of an effect on diplomacy, especially with certain people Isabella the worst. It wouldn't work in Civ V, and it needed plenty of work in its Civ IV state, so I don't miss that too much. If they took the time to actually fix it, and make it more interesting, then I'd be all for them working it back in. Corporations kinda had their own issues, but I actually liked them, and I do kind of miss that aspect of the late game. Something about the way economy functions in Civ V rubs me the wrong way in general.
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Simplicity is the ultimate sophistication.