The inclusion of multiplayer co-op and deathmatch modes changed everything about games. We knew that playing a game as fast and over-the-top as DOOM would signal a new era. I visualized what E1M7 would look like with two players shooting rockets at each other over a large room and it got me more excited than I had been since Wolfenstein 3D’s chaingun audio.
Its not to say Doom invented multiplayer, however most games were usually were pretty exclusive to a single screen. There was something almost magical, playing a game over a network for the first time, knowing there was another human being on the other side.