As you can see above, there's a new layout and redone art, new map-specific items in the store (no more generic wards!), Dominion-style speed shrines, and special altars with buffs to grab. There's also a new giant spider boss named Vilemaw who, like Baron Nashor, will grant a teamwide bonus when defeated.
The map is still in beta right now and is expected to be considered as such for a while, though the new version is being used in ranked Season 3 play already. Twisted Treeline's 3v3 player variant has almost always been part of League of Legends, but with this revamp LoL's millions of players have gained yet another way to experience the game.