Strictly speaking, if they had just packed Beyond the Sword with the engine, it would've been quite enough, thanks. I understand there is _some_ content from the Warlords expansion that didn't make it into ‘BtS’ and maybe I will look into that some day – maybe.
The appearance is familiar. The mechanisms are familiar. There are a few additions, like an espionage-as-civ production to chose from alongside money, research, culture. There are new buildings and so on. But the really exciting thing are the mods, and there are lots of them at www.civfanatics.com. Some are even included on the CD, like Final Frontier. But there are others, like MOO2Civ and Fall From Heaven 2 that completely change the appearance and feel of the Civ engine. Highly recommended.
I'd like to point out that I had some issues. I had to download the latest patches for BtS manually and install them, the autopatcher failed to do anything. I also had to download the DirectX August2008 patch from Microsoft, or the game would crash trying to start a game in some of the mods. But now it's fine and I am too busy playing to write much more here …