Warning: sort of long.
Tried out the Byzantines because they had a basic UA related to religion and dropped myself down to Prince to test out the new stuff in the expansion..
1) Dromons are very, very powerful compared to Triremes. Which was a good thing, because:
2) AIs seems to have gotten much better at combat, specifically naval combat. China was the nation right below mine, and while I was building culture buildings she took the opportunity to make a big fleet and launch an invasion. Luckily for me the turn before I had decided to get a few more Dromons to protect my borders, or I'd have lost my capital. Was a 5 land/8 ship fleet on Prince. I ended up crushing her pretty easily in the end, but it is only Prince after all.
3) Diplomacy seems a lot more stable now. After finishing off China 2 of the people I was not friends with called me a warmonger, but my friends didn't bat an eye. I have held steady friendships for over 250 turns now (on Epic) without a single breakdown. It's a nice change.
4) Religion seems very powerful to me. I made it a goal to get first pick of abilities and took the +1 production to fishing boats, which gave all my cities at least a +3. Used my UA to pick up the culture from Pastures (aiming for a Cultural victory, as those are my favourite) which amounted to +12ish, and all this came in before I had hit 100 turns. I do wish I had picked the option to buy units with Faith though, as after I built my Padogas it's been pretty worthless unless a city state quest comes up.
5) Espionage is a bit limited, but I like it as a hedge against runaway civs. I'm using my spies on Napoleon right now, as he's going up the military half of the tech tree and I'm going on the culture path. So I get free military tech upgrades without having to divert my beakers. I do wish there was a steal culture/sabotage building sort of option to give me something to do against people that aren't out-teching me, but that might be intentional that I cannot.
6) City states feel a lot more involved now. I actually aim for quests to keep my influence up, rather than throwing money at it. Faith ones might be a bit overpowered at the start of the game though. I like that the Military ones can give out specific UUs as well, makes your choice of city state relevant.
7) Love the new tech tree, and the archer line upgrades. Not so sure about the change to how hard cities are to capture, but I guess now that siege units don't have a resource cost it's okay.
Overall, glad I spent the money, would buy again.
--
Wind!
Falconnnnnnn PUNCH!