The world congress makes diplomacy more important, but it also rewards exploration. The first civ to discover every other major civ in the game, gets to convene the world congress . You can propose a resolution from a long list of options, all of which can impact the game drastically. If the proposal passes, the world congress can ban a luxury your rival depends on, embargo a civ from all trade or, my personal favorite, build a World's Fair to boost tourism. Making sure other civs like you is difficult, especially if you're winning, but it's still fun to interact with other civs in a more structured way. Again, the espionage system integrates well with the congress - you can choose to send a diplomat instead of a spy to their capitol to give you insight into how your rivals will vote.
Once you get to the modern era, the world congress transforms to the UN and can elect a world leader for a Diplomatic victory. Unfortunately, with enough city states on the map, a diplomatic victory is still basically an economic victory as you can buy their delegates by being their ally, and even purchase your rival's votes through the diplomatic trade. This new expansion certainly does not fix the artificiality of the diplomacy system in Civ V - opting instead to graft features onto an imperfect framework - but only a complete revamp would help at this point. Hopefully for Civ VI?
There are nine new civs in Brave New World, and it's great that most of them force you to play the game with a completely different strategy. Brazil's Dom Pedro, for example, is custom-made for a culture victory with each golden age - called "CARNIVAL!" - doubling your tourism for the duration, a huge boost in the late game. Venice is more challenging; playing Enrico Dandolo means you can't build settlers, but you receive double the number of trade routes and you can set up city states as puppets with the special Merchant of Venice unit. It's like a "one city challenge" hard-coded as a civ, and it felt really similar to how I envisioned historical Venice would play in a Civ game.
The new interesting civs and their unique units make up for the lack of new military units in Brave New World, but only barely. Building your own X-Com Squad is certainly a nice nod to the other dev team at Firaxis, but the unit comes far too late to make a huge impact on your game beyond a, "Hey, that's neat." The new bazooka unit completes the ranged unit line in the modern era, but Firaxis wisely believes the evened-out power curve of units in the last expansion did not need to be tinkered with further. You could certainly argue that's true, but it would have been fun to wage war with new units.
Bottom Line: The sweeping changes to the Civilization V ruleset are both refreshing and familiar. Each of the updates feels like a logical evolution and are seamlessly integrated with the existing systems. Brave New World feels like Civ V Complete.
Recommendation: No Civ fan should miss out on playing Brave New World and newcomers to the series won't be disappointed.