In ARMA 2 you can walk and jog while reloading a launcher.
In ARMA 3, once you hit reload with a launcher out, you can't move until it's finished.
Consider this scenario in A2: You peek over a ridge, take a shot with the launcher, turn around, and move back down the ridge while reloading the launcher to use again in another position or save for later.
In ARMA 3 this is not possible, you'd either get stuck on the ridge reloading, exposed to enemy fire, or have to move away before reloading.