An expansion which makes Civ V a whole new ballgame...
Huge downside however is that the game is still prone to crashes --too many and too often to be healthy, certainly as you progress and map activity goes up.
There's also a huge memory problem as you progress. That's one of the reasons why I use the 'Farm Replacement' mod, because, as of a certain point, after a restart, the farms no longer show which makes it a touch difficult to get a grasp on what tiles you've worked and which are still virgin ground.
What makes the BNW expansion so interesting is that now you've got trade routes you can use --both across land and over sea-- to either lay the foundations of a friendship with a rival civilization, or to ship goods between cities in your own empire.
The second real positive is that there's the addition of tourism as a ways to garner and even boost culture and thus try to achieve a Culture Victory by adding Great Works of Art to the musea and operas you've built which contain Great Works Of Art slots, or, later in the game, excavating archeological digs, unearthing cultural artifacts which you can keep (when a dig site is outside of your empire's borders) or add a sight-seeing monument in stead of the dig when that dig is inside your borders. The more culture you gather, the more influence you can excerse over other civs, paving the way for said cultural victory.
Third innovation is the establishing of the 'World Congress' (which later becomes the United Nations), after discovering the printing press, or when at least one one civ has encountered every other civ in the game.
In this context, City States get to play a bigger role, as any City State you can keep as your ally also gives you extra votes in the World Congress/United Nations, which is a huge help when a vote comes up.
Downside to this voting is the time it takes --and this is something that irks me over-all-- as it takes up to 90 turns (when playing on "marathon" speed), which, to be honest, is overly silly since it takes 1350 years (on "epic" speed) between the announcement of the World Congress and its first voting session.
I feel that this should be altered: set speed so that only one year goes by every turn. I mean, when adding a city too far away from your capital with little resources around can take 100 turns for a monument to get finished, that's between 1500 and 2500 years. Joking, right?
Anyway, keeping the Cite States as allies and not waltz over the lot to easily expand your empire is a bit of a dance on a not-so-tight rope, because A, you can keep citizens happy by trading City State luxury resources like silk, cotton or gold and gems and B it can also trigger rather unpleasant situations when a rival civ extorts money from a City State you swore to protect.
Now, becoming an ally of a City State doesn't make you automatically protect it, but you'd better do so, otherwise you won't get notified when said state is under attack by another civ. Once a City State is taken over, the possibility of that extra vote in the World Congress/United Nations is gone forever, as are of course the resources you traded.
Further added bonuses are new civilizations (nine in total), new World Wonders (eight in total) and twelve new units as well as a bunch of new buildings like Hotels and Airports and whatnot.
Upping the relevancy of City States, adding trade routes and tourism makes Civilization V a whole new game, really. But with that, there's a few downsides as well, or even downright cheats by the at times off kilter A.I.
For instance, when a prophet or missionary reverts a city of yours to another religion, a prophet or missionary of your own created in that converted city will belong to that other religion, not to the original religion you created. Now, when you do the same with other civs, their prophets and missionaries still are from their original religion, which, technically and playing "by the rules" is impossible. But 'impossible' isn't in the A.I.'s dictionary, I suppose.
Then there's archeological digs: most of them are visible, but there's invisible ones too, which are supposed to pop-up at a later stage. Except, they never seem to do so and stay dormant 'til kingdom comes. I've never seen one come to life anyway, and I've had games where I went as far as the year 2600 AD.
To be honest --and while I applaud the Gods And Kings and BNW expansions-- I still feel they didn't go far enough. I would have liked to see some more conversation possibilities and when you get denounced by a civ that they at least show you why, because a lot of the time such denouncements come out of the blue and take you by surprise.
I also feel the A.I. has gone a tad soft. Far less aggressive than before. Which sometimes can be a good thing, but I've had play-throughs where not a single civ showed but the slightest bit of animosity towards me.
That's about it in a nutshell. There's lots more, in fact, but you'll need to play the game in order to find out what the added possibilities are.
Me, I'm gonna peruse the mods section of the Civ Fanatics website and do things over, again. And then probably again...