Good news! The smooth rotation will come with time. Old habits are hard to break and as BOAC says, rotation technique is usually covered in depth on period one. A few more V1 cuts should smooth it out perfectly.
Your Flight Crew Training Manual shows a Typical Rotation - One Engine Inoperative pitch attitude of 10.6 degrees at the 5 second point after Vr. At 10.6 degrees you have a minimum tail clearance of 86 cm. Tail Strike Pitch attitude is only 12.5 degrees.
So your instructor wants to make sure you don't over rotate another 1.4 degrees into the tail strike zone while you're still on the ground. And obviously you didn't as your minimum tail clearance was 1.8 meters.
You'll going to love the 747-400. Without getting into an A vs. B argument, it's simply one of the best flying jetliners ever built. (with all due respect to the L1011).