Yeah, that was one of the biggest surprises for me. It really makes the piety tree (which I initially evaluated as questionable, since it contains little in the way of what I consider the most important things to be drawing from an early SP tree, like happiness) considerably more attractive. I'm not game-smart enough to know if it's actually good, but it's more useful than I would have guessed.To add to 2--if you want a religion and your Civ doesn't come with inherent faith bonuses, you will probably need to go Piety. I play on large with 12 Civs and the available religions get taken stupid fast. In G&K you could usually still snag one by T90ish or so; now they start dropping in about half that time.
The other big thing I'd wish I'd known is that the end-game ideologies are at least as much about the global politics that go into who has each one as they are about the actual bonuses inside of them. You give up a LOT more by going into an unpopular ideology (especially one that's favored by tourism leaders) than you gain by picking one that's maybe slightly better for what you're planning. This is especially true given that, despite each being allegedly less-well-suited for certain victory types, there's stuff in most of them for most strategies.