Eight people doing something correctly together, coordinating together is very time consuming, and just feels frustrating. If one key role dies such as a healer or tank, the fight goes sour. If someone misses a AOE dodge or does not know the fight it becomes a problem. This is the primary concern of the Duty finder in endgame.
Well, we'll just say if you cant handle coordnation with 8 people, you wouldn't have lasted 5 minutes in WoWs 20 and 40 man raids, or how about 64 man Dynamis runs when it came out in FFXI?

battles like those in coil that require strong coordnation are fun. I don't mind wiping 20 times as long as it isn't because people are being dumb. It's challenging, and challenge is a good thing. There's no such thing as too challenging unless it's impossible.