When I try to think of what enhancement's most iconic ability is, a few come to mind. Windfury Weapon is a definite contender, and three or four years ago it would have been my default response. Seriously, though, what is Windfury other than a glorified auto-attack with an attack power bonus and a lower chance to miss? Likewise, Stormstrike could be iconic, but it's essentially a melee attack with both weapons with an electric graphic and a debuff. It's enhancement's Mutilate or Obliterate.
To me, Maelstrom Weapon is the answer. If there's one ability that encapsulates what enhancement is -- a brutal melee class that weaves instant spellcasts between their weapon strikes to harm their opponent or heal themselves -- I believe it's Maelstrom Weapon. It's enhancement's Death Strike, Arcane Blast, Chakra, or Raging Blow -- abilities that, at a glance, offer an insight into what makes the spec different from other specs in their role.
There's a problem here, though. If Maelstrom Weapon is enhancement's iconic ability, why is it currently so awful?