If you see a player, what action of his would make say: 'Yea that guy is definitely skilled!'?
It matters not if it's in a dungeon, a BG, a duel, arena.
Here's mine: Knowing how to use your 'special situational CC'. A damn fine sight seeing a hunter fear a druid, a druid making the travelforming enemy druid sleep, a full unbroken repentance in enraged regeneration when he can't enrage out, a Holy Wrath/Turn Evil for Lichborne, a warlock actually banishing a tree druid, and it goes on. It either needed the player to be reactive or knowledgeable or what category the enemy can turn in. Sadly some of the stuff is gone though.
And there's Shadow Wordeath backfire to break CC. Not referring just to Poly here, that one's the easiest, you make him waste much time juking or already force a counterspel. It's quite a sight to see things like Repentace or Blind (instant cast) get broken, but not by chance, but predicted every time. Also there was Seal of Blood backfire for paladins, I often saved the Judgement for the same use, and it was quite a pity to see Seal of Blood go away, wrath pala skill...let's say it almost died with that day.
There's using the warr's spell reflect as a mage... to heal yourself with Polymorph.
Also there were those old abilities that were very uninteresting in TBC but got buffed in Wrath to have interesting effects, like Feint that lowered AoE damage, it was nice to see a rogue that knew to tank a Bladestorm or Divine Storm And that's before crazy sap-abusing tricks
At last, when I see someone fall victim to the Skill metagem (1% spellreflect chance) where he gets a strong CD reflected, but reacts quickly against it and still pulls out a win.
Long story short it's more around class knowledge, reaction, quick decisions and cooldown management.
What about you folks?