Alright. STUFF THAT HAS BEEN CHANGED/ADDED:
-Early game balance has shifted a bit, as Chariots get a 100% bonus when on the attack against axemen. This makes the early game less of an axe spam.
-Mounted units have been boosted quite a bit. War Elephants now come with Horseback Riding and require Ivory, not horses. They are awesome
-A new building called a Stable is available that gives extra exp for mounted units. With it and a barracks, you can get two promotions off the bat without Theocracy/Vassalage
-Vassals give you a happiness bonus in your cities, although with slightly increased maintenance. Vassals must give you resources that you ask for, and will always help you in a war. Their territory is treated as your own with regards to fortifications and such. Vassals can NOT declare war on their own. Vassals are like forced allies.
-The Palace was added. I can't spell the first part of the name (starts with an A

). It is a wonder you can build with theology. Basically, it becomes like the Vatican City of your state religion. Only one palace can be built, like other wonders. It gives all buildings of your state religion +2

. Its main function is that it acts like an early game UN. The amount of influence you have in the votes depends on how much of your population has the religion in question. It is possible to win a Religious victory this way, and the other resolutions are a bit different but similar to the UN resolutions (Stop trading with X, declare war on Y). You can not be victimized by the votes, though, if you have that religion as your state religion.
-Corporations were added. They're essentially late game religions. They take resources (and multiple resources stack, so 2 rice is 2x as good as 1 rice) and produce effects depending on the corporation. Cereal Mills, for example, takes food resources and gives a bunch of extra food to whatever city it's in. Corporations can also be used to make up for absence of oil or aluminum in your territory. Corps are strange balance-wise. In the base game, they are underpowered due to inflation and make you lose huge amounts of money. With Solver's Unofficial Patch, this bug is fixed (as well as a lot of other things), and it makes them a bit overpowered. I went into the .xml files and slightly lowered the corp income.
Corps can be spread to other nations, and this gives you a lot of money (as the corps add maintenance costs to the city). Since it's not your city, you aren't paying the maintenance costs, yet reap the money from your corporate headquarters (should be your Wall Street city). However, this gives the other nation that bonus, too. Last game, Mali spread Mining Inc to my cities, boosting my production and letting me make a spaceship earlier.
-Espionage was reworked. Commerce can now be diverted to Espionage Points (or they can be gained via other ways like buildings) and then spread out across other civs. Espionage points allow your spies to complete missions in enemy territory, and makes it harder for them to do so. It's an excellent part of the game, but broken without Solver's patch.
-The Great General was added. A Great General is spawned when you get a certain number of combat experience. It can do one of three things. It can work as a military instructor in a city, adding +2 experience to units made there. It can make a military academy (With the Military Science tech), which improves production speed by 50% for military units. Lastly, it can be assigned as a Warlord. It attaches to a unit and spreads out 20 exps to all units in that tile (20 xp in total, not 20xp for all of them. The unit that it is attached to can get some nice promotions. Tactics, for example, is +30% retreat chance (imagine doing this to a Flanking 2 Horse Archer for an 80% retreat chance!). One gives +100% experience in a battle, stuff like that
-Civs now have Unique Buildings. For example, the Russian Research Institute replaces laboratories, and gives you +2 free scientists.
-New units/buildings/wonders, as you know. Not going to list all of them, but some worth noting are the Great Wall (barbs can't enter your territory), paratroopers (24 strength infantry that can paradrop 5 tiles away) and more.
If you get an expansion, get Beyond the Sword.