I wouldn't call it an "I'll-get-you-next-time-Gadget" type thing. It felt more like he was constantly watching and manipulating, he even says at the end of the Drakuru quest chain

"As for you... I spare your insignificant life as a reward for this amusing betrayal. There may yet be a shred of potential in you. Be warned... When next we meet I shall require much more to justify your life. "

He spares you're life simply for his own amusement. He doesn't need to kill you because you pose no real threat to him. At the end of the LK fight in ICC he even says

"No question remains unanswered. No doubts linger. You are Azeroth's greatest champions! You overcame every challenge I laid before you. My mightiest servants have fallen before your relentless onslaught, your unbridled fury... Is it truly righteousness that drives you? I wonder.

You trained them well, Fordring. You delivered the greatest fighting force this world has ever known... right into my hands -- exactly as I intended. You shall be rewarded for your unwitting sacrifice. "

Throughout the entire expansion from the Scourge Invasion to the assult of Naxx and Ulduar, to the Nexus War, to the Argent Tournament, to the siege and final assault on ICC has only been playing into the LK hands. To deliver him the strongest most capable fighting force in Azeroth.

For an MMO its quite good storytelling. Its an arc that covers the entirety of the expansion and gives meaning to everything that has happened, and provides a logical reason why the LK just didn't destroy the Horde and Alliance Capital Cities, and why he let their armies run rampant across all of Northrend.

While from a greater literary perspective its a bit cheap, but from an MMO design point I think it was really well structured.