I already mentioned aggro in the Meleeing section. Although there is just one simple rule - only the Main Tank and offtanks should ever get aggro on a raid (with the exception of pulling the monsters), this is often a problem. The problem can have many causes.
Although the tank will do everything they can to make sure they have as much aggro as possible, it is not the responsibility of the tank to make sure that noone else will overaggro them; it is the responsibility of each person on the raid to make sure that they will not draw aggro off the tank. Melees should never use aggro weapons or taunt unless they are tanking. Casters should not nuke unless they are absolutely sure their nuke will not draw aggro off the tank. Critical spell hits do not generate more aggro than non-critical hits or resists, so don't use that as an excuse when you overaggro. Debuffers should not debuff the monsters (including slowing it) until the tank has enough aggro (although there will be exceptions to this).
As a rule of thumb, if the monster is attacking anyone else than the tank (and you can check that via the Health of Target's Target group/raid ability), this is wrong. That person who drew aggro off the tank (be it another melee, be it a nuker, be it a debuffer, be it anyone else) will need to change the way they do things, to make sure next time the tank can keep the aggro.
Drawing aggro off the tank wastes healers' mana (because they need to heal the person who overaggroed), but this is often the smallest problem that overaggroing causes. Because almost everyone is attacking from behind the monster, the monster will usually turn around when overaggroed, which will expose all the melees to ripostes; it will probably also move the monster to where the aggroer is, which can possibly expose casters to AE rampage or AE spells; it also forces the tank to move towards the new position of the monster, possibly moving the tank out of healing range. And even if this does not wipe the raid, the raid will not do much DPS until the monster is on the original spot again.
If someone draws aggro from the tank, it is necessary to let the tank regain it as soon as possible. The person who overaggroed should immediately stop attacking or doing anything else that increases aggro. Often people who get aggro this way try to run from the monster, because the monster would probably kill the overaggroer very fast, but this only makes things worse: the tanks can regain aggro best if the monster is not moving, because they need to see it and they need it to be in range of spells or aggro abilities. The person who overaggroes should stand still until the tank can get the situation under control again, even if that means the aggroer will die.
Sitting down or being low on health (approximately 30% or less) dramatically increases the chance that a monster will ignore people who have more aggro than you and will start beating you instead - because people who sit or are low on health are easy targets. Unless absolutely neccessary, please do not sit down when monsters are in camp.
Again, I can not stress this enough: not drawing aggro off the tank is very important and it is the responsibility of every person on the raid.
Monsters rooted by spells follow the usual aggro rules, with one exception: they ignore anyone who is not in their melee range (but they can summon someone outside their melee range directly to them). This makes it very difficult for the tanks, so usually using roots on raids is a bad idea, unless mentioned otherwise by the raid leader (perhaps to help with crowd control).
The same aggro rules applu to perma-rooted monsters, but there's nothing to prevent that root, so everyone (and especially the main tank) will need to be very careful about their positioning.