Did I miss something? When did X stop meaning kill straight after Skull, cause I think the whole of Azeroth may be trolling me... well I KNOW my raid team IS trolling me because I whinged about it and now they go out of there way to kill X last. But complete strangers seem to be trying hard to NOT kill X anymore.
And on that note, if you HAVE CC (I'm looking at you hunters and mages - others too but they seem to be the most reluctant these days) then USE it. Even if your tank doesn't ask for it you're allowed to sheep / trap / sap / sleep / mind control stuff. And NO knockbacks are NOT CC - save that s#1t for questing. Don't snare unasked but proper CC, go for it. And if your tank MARKS something with ANYTHING other than skull or X, then CC that bad boy. Don't stand around saying, we don't need CC. You don't have to be terribly skilled to do it. Your casting range is much further than aggro range and since Blizzard nerfed CC pulling there not much skill needed at all.
And assuming that your tank has some skill, don't CC the skull or the X. If (s)he's taken the time to mark the adds, he probably has a pulling plan in mind that may need a shield to bounce or a line of sight that you aren't necessarily aware of. You may like to argue the toss over whether or not the tank SHOULD lead the group, but the de facto situation at the moment is that they are EXPECTED to in most cases. And they ALWAYS have to be in charge of the pull.
It takes longer to wipe and run back than to just CC and kill a mob on its own.