BR stayed in the game, Covie Carbine stayed, Beam Rifle stayed, Spartan Laser stayed, Sword etc. So they are adding a few things...Halo changed a lot from Halo CE to Halo 2, and there were even changes from Halo 2 to Halo 3 believe it or not. It is still Halo and will still feel like Halo. It has new life in it and will continue to chnage from Halo 4 to Halo 5 and Halo 5 to Halo 6.
As far as the grenads, use the resupply mod to get more grenades off of dead bodies, or use the additional greande mod to carry 3 instead of two. You can also pick up grenades form grenade spawns on the maps. It will not be the end of the world