Originally Posted by
clampy
On normal you can jump around and join any boss with any group you want.
On heroic, if you kill a boss you are saved to a raid ID like before. This means you cannot swap around to different groups on heroic. Someone that hasn't killed any bosses can join your heroic run, even someone that has killed 4 normal bosses. But the moment you kill a heroic boss, you are locked into that heroic ID.
Once it is set to normal, these restrictions are removed until the raid is set back to heroic.