Pros:
New stuff is all really cool (more below)
World Congress creates more interactive world diplomacy
New Culture victory is decently hard, but also probably the most fun victory yet, and one that I hope they build off of in Civ VI.
Trade routes are a very nice addition to the game, creating a more active style of costal play.
Great works are also a much more active style of culture/tourism, forcing you to think a little more about how to get culture. It's nice when they take away flat bonuses (+1 gold per river/costal hex, +large culture for having a building) and give more dynamic bonuses with additional options on how to use them (using trade routes to either gain gold or grow your empire, working guilds to create great works to store in your buildings)
Archeology is another cool example of a more active mechanic this expansion brought (create archeologist, send him out, create either a landmark/artifact vs create a building and experience culture)
Ideologies provide a lot more policy choice late game, and provides actual gameplay incentives to get others to switch to your ideology
More city state quests
The new civilizations are pretty fun and probably better designed as a whole than ones from vanilla.
Cons
Tall is now a dominant strategy, which is kind of boring at times
Science is just as dominant as ever, and is kind of even moreso than before the engine that drives all victory, even cultural victory
Warmongering is perhaps overly difficult without serious diplo penalties, especially if you take cities by force and not through peace treaties.
On that note, some games can be overly peaceful if you don't do anything to provoke the AIs. Although, Alexander is still just as big a jerk/warmonger as ever.
Some people really hate the world congress. Not sure why, although there probably should be more options and more useful options early on.
I think it's well worth taking. Any pros and cons I missed? These seem to be the big ones that I can recall.