All the problems, like
  1. People not being allowed to run about like headless chickens during mechanics.
  2. Not being able to stack all tanks and healers to cheese the fights, albeit at twice the kill time or slower.
  3. Actually caring about playing their job well, instead of just... not wiping.
  4. There being momentum and excitement in a fight through optimization, rather than just variance between complete allowance of blunders and less blundering permitted.
  5. Not being permitted to afk through a fight or duo what should be 8-man content.
Just look at them! The horror!