The paladin skill Exorcism has undergone some changes during the past few years. Originally it was only castable on undead and demon targets, and had a 1,5 sec cast time. This meant it had a very limited use and so very few paladins actually used it in their regular dps/tank rotations. It was one of them "what is this really good for anyway" kind of skills. Little more than half a year ago, Blizzard decided to change this however and made the skill significantly better. They made it so you could cast it on any kind of mob, but it would automatically crit when used on a demon or undead target. Also, they removed the cast time, and made it instant.
Suddenly, every paladin had a new, really good skill to use. In fact it was so good, Blizzard soon decided it shouldn't be usable against other players (i.e pvp), but only against mobs. The usage of Exorcism was soon rooted into the heart of every paladin. It became a standard spell in every retri- and prot-paladins rotation. But then Blizzard changed it again, and this time back to the worse.
Not all the way back to uselessness thank god, for example it is still usable on any kind of mob, and will crit if the target is undead or demon. However, they did reimplement the cast time of 1,5 seconds. They also changed the retri-paladin talent "Art of War", that originally would reduce the cast time of Flash of Light to instant when critting with a melee attack, to also reduce the cast time of Exorcism to instant. This meant retri-paladins could still get the instant Exorcism, but not Prot- or Holy-paladins.
For Holy this wasn't a problem, they hadn't used it much anyway since well, they're supposed to heal, not dps.
For Prot paladins on the other hand this arised an issue, which many of them don't even realize is there. If a prot paladin wants to use Exorcism they will have to cast it over 1,5 seconds. For pulling mobs, this is still a decent spell. Especially in lower levels before you have spells like Avenger's Shield. The problem is when a prot paladin uses it in close combat, in melee, like if there hadn't been any change to it and it was still instant. Why is this is this a problem?
Because you can't block, parry or dodge when casting. So if you as a tank are currently being hit on by 5 mobs, and start casting something, like say a heal or Exorcism, it will greatly increase the dmg you're taking. A googling on the matter will quickly turn up plenty of posts that say you can't dodge, parry or block while casting, but none that say you can. Another problem is that this is only an issue for protection paladins, since they are currently the only tank class able to cast anything while tanking. Oh well, and bear druids. But since they shape sift out fo bear form when casting a spell, it's a little more obvious what a bad idea that is. This means this problem isn't something often discussed among tanks, and the vast majority of people don't even know it exists at all.
Not only that, people go ape shit crazy when I mention it to them. "Could you please not cast while tanking, you take way more dmg that way" usually gets the response "no you're wrong" or "shut the fuck up". Such lovely people out there. If they've never heard about it, it's not true, right?
They don't even stop a second to think about it, or say "ok, I haven't heard about that before but I will check it out to see if it is true". Don't people want to learn more about the game? I'm not saying you should believe everything one says to you, but if there is even a remote chance that this is true, why not make sure you know about it? Maybe it doesn't matter much in the heroics of today, but it definitely will matter if you ever expect to raid. Cast something while tanking a boss and you might actually die.
I didn't know about this either, until someone told me and I tried it out for myself. You can't know something before you do. And just to make things prefectly clear - Do not cast something while tanking please!