The Halo 1 animations are as juicy and fluid as they are partly because, without having to be a snappy response to a button click, they could have a huge amount of lead-in. The amount of time between a Halo 1 elite's melee animation start and the melee actually making contact would be infuriating for a player.

Similarly, Halo 1's elite animations didn't have to worry about the characters strafing around and rotating in unexpected ways during the actions. Halo 2's did.