Sid Meier's Civilization V: Gods & Kings
There are two big new features in Gods and Kings: religion and espionage. But neither of them change the game as much as the expansion's general unit additions and modifications. Actually, Gods and Kings feels less like an expansion pack, and more like a large patch -- definitely good to have, but probably not worth paying too much for.
The religion system is a little different than what I expected: specifically, it's not all that powerful. There are some really cool bonuses you can get from spreading your religion (similar to the bonuses from selecting cultural policies), and spreading your word can have a subtle but real effect on diplomacy. But you can't just win, or even really manipulate your opponents significantly, by sending out missionaries and prophets. Since religious bonuses are picked first-come first-serve, the most important part of a religion is founding it early; after that, there isn't necessarily much to it.
Espionage is more interesting, especially since it becomes more relevant in the later game. Starting with the Renaissance era, every technological era gives you a new spy to work with, and you can send a spy out to enemy cities to steal their technology, or keep them in yours to protect against technology theft. You can also have a spy rig elections in a city-state to curry its favor, or stage a coup in a city-state allied with someone else. But arguably the best part of spying is in its side-effects -- when your spy is set up in an enemy city, you have a chance of being alerted when the enemy is plotting something. If they're plotting against another AI, you can even choose to tip the target off. Espionage is a welcome way to finally gain some insight into the AI's twisted scheming.
Aside from those, there are some new civilizations and new civ-specific talents, and even reorganization in the tech tree. But some of the biggest changes are to units and upgrades. There are new World War I-era ground and air units, new upgrades for ranged ground units, and - my personal favorite - new mechanics for naval units, which allow some ships to capture coastal cities. Finally, my obsession with churning out an overpowered navy pays off.
Like I said, Gods and Kings has some great improvements to Civ V, but they all seem iterative rather than transformative. This is unquestionably better than the original, but not by all that much.
Better than: Sid Meier's Civilization V
Not as good as: a $30 game should be.
The recent patch is also a general improvement: although there are still some inexcusable performance issues in multiplayer.
Progress: Finished a couple of campaigns