Civ V on its own is garbage. Brave New World somehow transforms it into almost being on par with Civ IV minus the excessive micromanaging. Civ V is unplayable to me without BNW.
No really! It fixes the vast majority of the inexcusable crappiness of vanilla Civ V.
Brave New World makes vital tweaks that bring much needed complexity to Civ V. Particularly where victory is concerned. The two biggest reworked aspects are culture (totally redone) and the role city-states play in a diplomatic victory. Trade is now present (finally) changing how the game is played substantially. Important revisions were made to civ AI so they now behave a certain way and no longer randomly declare war on you for absolutely no reason.
BNW is so good, the only thing its missing to make Civ V perfect is balanced late game melee units. In other Civ games, the philosphy behind melee units is they're either defensive, offensive, or balanced. Civ V attemps to do this, but screws it up.
The highly mobile offensive units used for invasion become quickly outdated by the time you reach cavalry and don't get better again until modern armor.
The defensive units used to repel an invasion are even worse. They completely lose their usefulness once you reach lancers and never get good again.
The middle of the road units are too useful. Mostly since G&K added a new infantry unit (great war infantry) granting the balanced fighters more flexibility and reliability. This allows the player to upgrade them more quickly and frequently than other unit types. No new units were ever added to the offensive/defensive categories, so they're at a distinct disadvantage from the long gaps between upgrades. Not to mention offense/defense units are weaker than mid-late game infantry. Its embarassing to know armored units NEVER get stronger than foot soldiers until modern armor. That's an end game unit! As of BNW, basic infantry are as strong as tanks and can out gun helicopter gunships. Two things that normally would flatten infantry in real life. Its the only thing has left to straighten out before it's as good as its ever gonna get.