Religion isn't meaningless diplomatically in Civ5, it's just that you're not automatically hated for following a different religion. If another civ follows a different religion and you spread your to them, they won't be happy. But if they don't have any religion, you'll get a positive modifier for spreading, because they'll get to share in the Follower Beliefs. Whereas in Civ4 every religion was identical, in Civ5 they are always different, with different bonuses. If another civ's religion allows you to have Cathedrals, you might want that in your cities. But if the Follower Belief isn't so great, you'll want that religion to stay out.
Civ5's religion system is far more complex than that in Civ4, basically. It may have drawbacks, and if you could of course still prefer Civ4's religion, but if you're looking for complexity, this is a front on which Civ5 clearly wins out.
OK, that sounds pretty good. While I like religion in Civ 4, I find it a bit simplistic and over the top diplomatically. It's good to see there's more variations in the system now.