Economy:
It's really clever. I *really* have to think about what buildings I want, and whether I can justify their existence. In Civ 4 I pretty much just built everything... if I didn't need more military I'd build a building, if I didn't need a building I'd either build military or just build a building that would be 'handy' but I didn't really need.
The maintenance is a killer. I could build a Colosseum to sort out my unhappiness problem, but I just daren't because of the 3 maintenance which seems like much bigger deal than building maintenance in Civ 4, because it's not just gonna be a case of my science dropping to 90% for a few turns...
I really have to think about not only what buildings I need but when or if I can justify paying for them...
Systems like religion / espionage may have been removed, but I'm being forced to consider core things a lot more that I just kind of did without that much of a concern on Civ 4.
It feels like they've basically taken city maintenance and transplanted it into extra unit / building maintenance. I'm free to build as many or few cities as I like, which is quite a liberating feeling, without obsessing over their maintenance. But that limiter is still present in the form of expensive buildings and units.