ShahJahanII
Homesick Alien
Just a few thoughts on the subject:
Songhai:
The Songhai's Unique Unit lacks the penalty for attacking cities from the Knights they replace. Although they are now cheaper, the fact that cities are much stronger and armies need siege units to soften them up means that the Mandekalu Cavalry have been seriously nerfed.
Babylon:
Here's one a lot of people have been talking about.
1. Great Scientists have been nerfed overall. An obvious nerf to Babylon's UA, their strongest component. Although, they will do better at early-game research since academies have been boosted slightly.
2.The Bowmen are now obsolete a lot quicker. While they used to be a great offensive unit, they aren't that much better since any civ needs only to get to Construction as fast as possible for Composite Bowmen.
3.The Walls of Babylon now provide extra HP to the city, a boost that is probably a lot more useful on higher difficulty levels.
Iroquois:
The Mohawk Warriors now have a 33% combat bonus in Forests and Jungles.
Rome:
Ballistae (as well as all catapults) no longer require iron and are much more useful overall.
India:
Population Growth doesn't seem as powerful as before since unhappiness is easier to deal with.
Korea:
1.One of the biggest, Catapults retain the "200% Ranged Strength vs. Cities" promotion when upgraded to Hwachas, making them much stronger than regular trebuchets.
2. Since you can now upgrade Triremes to Turtle Ships, which can then be upgraded to Ironclads (that can now enter oceans), they are now much more useful throughout the game.
America, Spain, the Ottomans, France:
Swords now upgrade to Musketmen, meaning you no longer have to build Minutemen, Tercios, Janissaries, and Musketeers.
Ottomans:
1. Already mentioned above.
2. Pikemen now upgrade to Lancers, so you don't have to build Spiahi.
3. Naval units are much more useful, so the discount on them is always appreciated.
4. Barbary Corsairs now gives the Prize Ships promotion to all melee ships, which can double the size of your navy in some games.
England:
1.Longbowmen upgrade to 2-Range Gatling Guns. Very useful.
2. Ships of the Line are more useful.
3. An extra spy is nothing to scoff at.
Greece:
Influence is gained a lot easier via quests. Using Greece's ability with Patronage unlocked on a city-state following your religion, you can make it so your influence never drops (unless, of course, a spy tampers with it).
This wasn't meant to be a full guide, just some things I noticed. So please share your thoughts and help add on to this list.
Songhai:
The Songhai's Unique Unit lacks the penalty for attacking cities from the Knights they replace. Although they are now cheaper, the fact that cities are much stronger and armies need siege units to soften them up means that the Mandekalu Cavalry have been seriously nerfed.
Babylon:
Here's one a lot of people have been talking about.
1. Great Scientists have been nerfed overall. An obvious nerf to Babylon's UA, their strongest component. Although, they will do better at early-game research since academies have been boosted slightly.
2.The Bowmen are now obsolete a lot quicker. While they used to be a great offensive unit, they aren't that much better since any civ needs only to get to Construction as fast as possible for Composite Bowmen.
3.The Walls of Babylon now provide extra HP to the city, a boost that is probably a lot more useful on higher difficulty levels.
Iroquois:
The Mohawk Warriors now have a 33% combat bonus in Forests and Jungles.
Rome:
Ballistae (as well as all catapults) no longer require iron and are much more useful overall.
India:
Population Growth doesn't seem as powerful as before since unhappiness is easier to deal with.
Korea:
1.One of the biggest, Catapults retain the "200% Ranged Strength vs. Cities" promotion when upgraded to Hwachas, making them much stronger than regular trebuchets.
2. Since you can now upgrade Triremes to Turtle Ships, which can then be upgraded to Ironclads (that can now enter oceans), they are now much more useful throughout the game.
America, Spain, the Ottomans, France:
Swords now upgrade to Musketmen, meaning you no longer have to build Minutemen, Tercios, Janissaries, and Musketeers.
Ottomans:
1. Already mentioned above.
2. Pikemen now upgrade to Lancers, so you don't have to build Spiahi.
3. Naval units are much more useful, so the discount on them is always appreciated.
4. Barbary Corsairs now gives the Prize Ships promotion to all melee ships, which can double the size of your navy in some games.
England:
1.Longbowmen upgrade to 2-Range Gatling Guns. Very useful.
2. Ships of the Line are more useful.
3. An extra spy is nothing to scoff at.
Greece:
Influence is gained a lot easier via quests. Using Greece's ability with Patronage unlocked on a city-state following your religion, you can make it so your influence never drops (unless, of course, a spy tampers with it).
This wasn't meant to be a full guide, just some things I noticed. So please share your thoughts and help add on to this list.
to 
faces means it is also a touch easier to exploit Persia's UA. Religion(if you can found one) and mercantile city-states are the biggest potential boons here.
