I think the mistake they made was this: In earlier Civs, late game bog-down was caused by SO many cities and SO many units. THAT's what became tedius to me (that, and microing workers to clean up polution). Now, in Civ4, there are usually fewer cities, units, and no polution on the screen, so they fixed the problem as far as I'm concerned.
But THEN, they made their big mistake: Making modern technologies researchable in 2-5 turns, which causes units to become obsoulete before you can even use them

, and worse yet, the era rushes by so fast, you don't even have time to actually enjoy the "new" end-game. And since, in that era, you can see most the map, and often need to "travel" units to get to those parts of the map, units are often even more obsoulete in this game by the time they get to their destination, than in past Civs. Even on the epic setting, it just makes the first 3/4rths of the game "bog down", and makes the end game about right
If they could just add, umm, I don't know, about +20% to modern era tech research time, and maybe add about +20% more resourses to build the SS parts, things would all come out about right in my oppinion
