Which I thought should have been done in the original game. I felt religion was one of the best features to be added in Civ4, it just needs some improvement. I was also hoping on seeing Corporations expanded on to make trading more robust and dynamic, but those were cut too.
Well no tech trading is certainly a big one for me. That's always been an important decision for me to make in the game, even as far back as Civ 1. Do I trade a tech that might lose me a Wonder in order to catch up with a civ I'm falling behind from, or one that will allow a potential enemy to gain an advantage over me? Plus the Civ wide happiness and the removal of city health meant that I didn't have to think very much about what to build in a particular city. Having culture spread one tile at a time meant I didn't have to think about how to develop my city area, I just had to respond to whatever terrain happened to appear at that time. Having my troops be able to miraculously find a boat that would transport them across the ocean at any point meant that I didn't have to decide which would be the best place on my landmass from which to launch an attack on another continent. Plus I read lots of complaints about the diplomacy in the game, how it was virtually impossible to develop any sort of meaningful relationship with another civ. Even Jon Schafer, in an article I read recently, mentioned how woefully inconsistent the other civs could be. Those and a few others I can't recall at the moment are the complexities I felt were missing.