Determining visibility

My memories of 4th are hazy, and bleed into the tail end of 3rd. But in the before time...in the long, long ago...I remember some pretty brutal games. Consolidating into assaults (and doesn't time come full circle). Monoliths that could never be killed with glancing hits. Waves of Tyranids and Khorne Berzerkers crossing the whole table and assaulting turn one...Eldar armies spawning from midfield...Dark Eldar killing you without ever leaving their vehicles...Assassins that could use Hit&Run before you got to hit them in assault...

My memories aren't super reliable from that time period...(our game store was right next to a head shop, for good damn reason)...and I'm not sure if I'm confusing editions, or if some folks were outright cheating...but my memories of 4th are less 'robust rules set' and much more 'cause for sweat soaked trauma nightmares.'