Not saying it is easy and I have already stressed the point imho it was more skillful than Sigurdsson's effort how he took it on first time. However as people have pointed out, usually a Charlie Adam type player would take a touch then balloon it into the air as if you were going for the crossbar challenge, this would make it easy to miss any players inbetween. (whether it's on target or not is a different question) Rooney's effort could have easily just hit the ref and that would have been that.
Go to the park and try and do some lobbing practice from the halfway line if the ball bounces on and around the penalty spot you would have easily cleared the defenders and see how many times you can but the ball between the sticks. That's all Wayne had to do, no one was guarding the net is what my point is. The style he did it to keep the trajectory low and hit the ball as it is moving with pace was off charts though.