Soon to hit the Public Test Realms, Mists of Pandaria's 5.1 patch is the first step in what will be an ongoing conflict between the Alliance and the Horde, escalating and spreading across the new continent with each major update. The first wave will focus on the eastern coast and the Krasarang Wilds region, where Alliance players will represent the new Operation: Shieldwall faction, and Horde players the opposing Dominance Offensive.
Earning rep with these new factions will be primarily accomplished through daily quests that involve defending and attacking outposts, killing opposing faction players and NPCs, and capturing strategic objectives. You'll also be able to spend the currency from these accomplishments to upgrade your faction's bases and hire NPC guards.
Also coming is an update to
Pokémon mode pet battles in the form of Battle-stones, allowing you to upgrade the quality of any pets from Poor to Uncommon, and Uncommon to Rare. Whether this will retroactively award a leveled pet the stat points from being a higher quality isn't specified, nor whether it will affect fixed quality pets like the ones awarded to Blizzcon attendees and purchasers of the Starcraft 2 and Diablo 3 collector's editions.
Lastly, two new Brawler's Guilds have arisen in the seedy underbellies of Stormwind and Orgrimmar. The first rule of the Brawler's Guild is that we don't talk about the Brawler's Guild. But what I'm doing now is technically writing, so I'm sliding by on a technicality by following the letter, but not the spirit, of the law. It sounds similar to Oblivion's Arena quest line, challenging players by "testing their solo PvE mettle against some of the toughest creatures found in World of Warcraft." You'll also be able to spectate other brawlers' matches. But be aware, as the patch notes specify: "If this is your first night at Brawler's Guild, you have to fight."