Reportedly, the difficulty level "won't be toned down for casual gamers," which means Team Ninja is making the game "more accessible" in a different way. In addition, it packs competitive and cooperative multiplayer, with at least one mode supporting up to eight players. There's also a possibility of Move support. The weapons store is gone, in favor of weapons that "upgrade over time."
According to RatedRSuperstar, "Blood is back" as well. Having seen the purported scans, that's an understatement. Imagine a ninja covered in melted Twizzlers.