Really upset about the lack of the Plasma Rifle. That is one of Halo's most iconic weapons, and the replacement just doesn't look all too special.
Still, I'm glad that the carbine is returning (I love that gun!) and the Beam Rifle as well (I'm sorry, but I hated the Focus Rifle). The Grav Hammer is probably exclusive to multiplayer, since there probably won't be any Brutes in the game, and seeing the Concussion Rifle was a very welcome surprise.