Of course they're obvious. He didn't say that he had no idea that the change in eras was happening. He was saying that you never got a sense of the change from the gameplay. Surely you remember how it was in Civ III & IV, where you knew you were in the medieval era because you were pushing back the English invaders with macemen and knights. No screenies were even necessary. I suspect the lack of "era flavor" is primarily due to equal parts lack of era specific music, and the "fewer units" design theory. With only half a dozen units, some of which might be un-upgraded spearmen, you just don't get the same "texture" you did with eight knights and twenty macemen bulldozering away the Aztec scum. There are probably other subtle things missing that I haven't noticed are gone, but I haven't gone back to IV yet to check. Regardless, whatever it is that is missing, it is very obviously missing, and it's not something that a popup saying "you are in the classical era!" can take the place of.