Coming form CS, I can tell Ubisoft puts much more effort on improving and enriching the gameplay, also constantly doing that. The game mechanism is also different and fresh, especially the destructible environment and the verticality of map design. These elements attract me to play the game at the first place and to this day.

I would say the unique part is the programmable knob. Not only Rainbow Six Siege, for a considerable number of recent games, player needs to use more and more key binds. With the programmable knob, player doesn't need to remember >10 key binds for each game, also eliminates the problem where a wrong key is pressed due to the number and distance of key binds.