Also, instead of just having a religion in a city or not, it counts the number of people in a city that follow religion X, Y, or z. So, while it's presumably possible to cram every world religion into one city (as per civ 4) it's certainly difficult.
Plus, there are some faith tenets that give you bonuses for having more people of your faith in any given city. Thus, there is more of a reason to try to keep your society closed from the influences of foreign religions.
This is radically different from Civ 4 where the best thing to do is, arguably, cram as many religions as you can into your cities.
The new wonders and civs are pretty cool too (though Sweden's special ability appears pretty underwhelming).