Some of these discussions among higher level players astonish me.
I'll tell you something. I started playing this game a couple of weeks ago. I just reached Level 29 yesterday on my Marauder and this is my first post.
I know to burn down the easier mobs before taking on the bosses through my leveling missions. How do higher level players get to 50 without knowing this? Amazing.
If you play long enough you will see the most astonishing things, and when you think you've seen it all... you will see something that will prove you wrong. I could spend the entire day listing dumb things people say and do at level 50.
I believe the issue is that most people now outgear most of the content, with people in Campaign or Dread Guard gear running HM flashpoints tuned for Tionese. So they get used to easy mode runs where you can just jump and throw AOEs into a pack including one gold or more, several silvers and a few trash without too much trouble (thanks to the healer working his *** off though).
Once you have taken bad habits here you find people focusing on the gold in a pack while the other mobs with low HP can still cause havoc - basically acting against everything you're taught while leveling. Then you will find the very same people running HMs on their alts in Tionese still rushing in packs without using any CC and wondering why they got crushed.
I remember when we were starting HMs months ago with barely a few purple mods from dailies and we had to pay attention how to approach these situations using CC, LOS etc. Try to mark and CC anything today, you will be laughed at for wasting everyone's time.