Adds. Those guys that spawn during fights, that do stuff and things. Dealing with them (usually by stabbing them in the face) is not a polite suggestion, as many players seem to think it is. Even during Wrath, I think most players got that adds = kill, but the time frame in which they needed to be dealt with was much more lax. It was okay if you finished your rotation before swapping, finished eating that last bite of pizza before taunting, or ran in circles crying about healgro for a few seconds. It’s not okay anymore. When they exist in a Cataclysm fight, dealing with the adds is almost always crucial to success. Taking care of them, be it via stabbing, sheeping, or some other mechanic is neither optional, nor something you can do at your own pace.
Just going through heroic bosses: Commander Springvale, ignoring adds will wipe you. Drahga Shadowburner, ignoring adds will wipe you. Erudax, ignoring adds will wipe you. Lord Obsidius, Corla, Lockmaw… you get the idea. Even the easiest adds, which don’t have a wiping mechanic directly tied to them like Rom’ogg Bonecrusher or Baron Silverlaine need to be delt with. If left unchecked, they significantly increase damage intake, which in turn drains more mana from the healer which will, you guessed it: wipe you.
Most fights exist somewhere between these extremes, in that a couple of slip ups will probably be recoverable, but they have nasty effects that will push the fight outside the realm of doable if left to run rampant. Again, this is not to say that most players will ignore adds (though plenty do), but rather that they don’t have enough respect for them. Don’t finish that cast, don’t get that last strike in, don’t wait for them to spawn/come to you, don’t unnecessarily pull aggro on them, and most importantly don’t assume someone else will handle it. Whatever it is you’re doing, dealing with the adds is very likely more important. So… deal with them. You’ll win more and get yelled at less.