Also I don't see what makes threads like this fair when all that happened was one or two people mass down voting Korea. OMG I can't enter ocean squares with Astronomy, its the end of the world!
The thread is just meant to be an assessment of the board's opinions on UB/UU/UI combos. It's not a perfectly objective ranking of every civ's uniques. I think you are taking this a little too seriously.
As you said, a human player can use Turtle ships more effectively. But as a human player, if I see you hoarding EXP and then hiding in your cities, I'll just ignore your cities by sea or attack more from land. And it isn't like I don't get EXP from attacking, too. And yes, Korea has Frigates, but I have Caravels that can guard my Frigates from the ocean. All this concludes to Korea being better at defense, which I've already admitted.
The simple fact is without the UA, I and many others find Korea's UUs to be not compelling enough to make me want to select them as my civ. An era-specific increase in defense in the form of UUs without promotions that carry over isn't very appealing to me. Korea can defend, but I'll be back later when we are on an even playing field again. I applaud them for being unique, I just don't care for them.
America: B17, Minuteman 22
Arabia: Camel Archer, Bazaar 21
Austria: Hussar, Coffee House 14
Aztecs: Jaguar, Floating Gardens 21
Babylon: Bowman, Walls of Babylon 9
Byzantium: Cataphract, Dromon 20
Carthage: African Forest Elephant, Quinquereme 21
Celts: Pictish Warrior, Ceilidh Hall 16
China: Chu-Ko-Nu, Paper Maker 24
Denmark: Berserker, Norwegian Ski Infantry 18
Egypt: War Chariot, Burial Tomb 5
England: Longbowman, Ship of the Line 23
Ethiopia: Mehal Sefari, Stele 25
France: Foreign Legion, Musketeer 23
Germany: Landsknecht, Panzer 19
Greece: Companion Cavalry, Hoplite 11
Huns: Horse Archer, Battering Ram 18
Inca: Slinger, Terrace Farm 16
India: War Elephant, Mughal Fort 12
Iroquois: Mohawk Warrior, Longhouse 21
Japan: Samurai, Zero 12
Maya: Atlatlist, Pyramid 25
Mongolia: Keshik, Khan 22
Netherlands: Sea Beggar, Polder 22
Ottomans: Janissary, Sipahi 18
Persia: Immortal, Satrap's Court 23
Polynesia: Maori Warrior, Moai 10
Rome: Ballista, Legion 24
Russia: Cossack, Krepost 17
Siam: Naresuans Elephant, Wat 18
Songhai: Mandekalu Cavalry, Mud Pyramid Mosque 21
Spain: Tercio, Conquistador 25
Sweden: Hakkapeliitta, Carolean 21
+Netherlands: I'm a big fan of both of these uniques. I love how the Sea Beggar starts with so many extra promotions. The Polder is extremely powerful if you get a start that can make good use of it. With start bias, that is likely to happen. It adds a nice flavor to the aesthetics of your land, to boot.
-Polynesia: I have to agree with what others have said. The Maori Warrior ability is nice in that it carries over, but it could use an increase in effectiveness. And whenever I have used the Moai, I've felt that they have hindered me more than they have helped. A little extra culture isn't enough to warrant me giving up other tile improvements I desperately need in the early game. And a typical Polynesia playstyle means that your cities are going to be in places where there already isn't a lot of land available.