Firaxis promised that BTS would be the biggest Civ expansion, and they didn't dissapoint.
After Warlords was a little underwhelming, BTS needed to add a lot to CIV, and it does just does that. 10 new Civs are added, each with their own leader, an additional 6 leaders are added to existing Civs and there a 5 new wonders, the most noteable being the Apostolic Palace.
The reason that the Apostolic Palace is the most noticeable is that it has a whole gameplay mechanic set around. Bascially, when built, global elections are triggered that work in much the same manner as the UN. Every so often, you will get to vote on things such as Trade Embargo's, stoping wars, or even asking other Civs to join you in wars. Unlike with the UN however, instead of the number of votes you get being based around population, when built, the Palace will take the state religion of the civ that built it. Only Civs that have that religion present in one of their cities are able to vote, and the number of cities that have that religion effect how many votes you get. At first, the Apostolic Palace can be confusing, but once you get used to it, it does add an interesting strategic element to it.
Corporations also find themselves included in BTS, and is the second of the three major additions in this expansion. They work in a similar way to religions, in that you spread them around by using an executive to spread it to other cities. Instead of techs founding Corporations like happens with religion, you use a Great Person to found a corporation. Corporations bring numerous benfits to the user, some grant resources, some add culture and some add food. There is a price to pay, whilst having corporations present in your cities will give you income, they also increase maintenace costs. It actually seems more beneficial to spread corportaions to your rivals then your own cities as it cripples their economy whilst still improving yours. In this sense, it does feel a little broken, and in need of patching, but it is still a great addition.
The final major addition is espionage. Before, the use of spies had been tedious, and, on the whole, pointless. Now, however, this element of the game gets a complete face lift. Now, spies can be built when you research Alphabet. Spies also have a whole new variety, from stealing technologies, to poisoning a cities water supply. Spies are no longer useless, but essential to both your Civs defence, and offensive strategies. There is also now an espionage slider, that works in the same way as the cultural slider, diverting some of your treasury to watching your foes. It is also possible to adjust how you monitor other civs, so it makes sense to invest more in watching you rivals, and less on those civs that offer little threat to you. There are some problems though. On marathon speed, there are some balance issues for some of the spy missions, and the new Great Spy unit isn't that great, but overall, espionage is a great and essential addition to Civ 4.
There are also some a few more additions. Random events have been added. Every so often one of these may pop up, with the effects being either good or bad. There are also Advanced Starts, that starts the game in the modern era, and you also now have the ability to mix and match you leaders with different civs.
This is an essential buy for any Civ 4 owner. And don't worry if you don't have Warlords. BTS contains all of the Warlords content, with the exception of the rather poor quality scenarios, so this is the only expansion that you will need. With a few fixes, that can be expected in a patch, this is comes highly recommended.