Serge Ibaka and Dwight Howard are both wildly talented shot-blockers, and the former presents a good model for the latter to aspire to. While Dwight Howard is thought of as a "center" and Ibaka as a "power forward," they are both of similar size and quickness.
I would argue that the game's best center is largely defined by that quickness. Dwight's strength is impressive, but he's short for his position. Though listed at 6'11", Howard checked into the 2004 NBA draft at a clean 6'9" in socks.
He has not appeared to have grown since his rookie season, and that "in shoes" 6'11" listing never went up. So I would posit that Dwight, like most Americans, did not grow after age 18. You may hear Howard constantly referred to as a "seven-footer," but the description is more wishful than factual. You can hear Dwight cop to his undersized status during the famed Hakeem Olajuwon workouts.
If only Dwight Howard's true height was widely understood, we might give him more credit—as opposed to attributing so much success to "size." Dwight's a brilliant defensive player and it extends beyond his physical ability to cover the most floor space in the league. Defensively, he's the triumph of quickness over size, but he also represents the triumph of intelligence over brute strength.
Howard's almost never out of position on pick-and-rolls, and he rarely falls for pump fakes. Blocking the ball out of bounds is about the only misguided defensive tendency Dwight possesses.
There is little difference between Dwight Howard and Serge Ibaka in how they block shots. Both cover a deceptive amount of space, and both can come flying from the weak side in a flash. The difference is that Howard blocks shots while being defensively responsible in other respects.
Serge Ibaka, in contrast, has some defensive flaws to go along with that astounding 4.8-swats-per-36-minutes measure. He's still a plus-defender on account of the rim protection, but he has a long way to go before contributing value along with the rejections.
While it is difficult to find tangible evidence of Ibaka's occasional over-exuberance on defense, I can point to his foul rate. The Thunder big man has averaged 4.3 fouls per 36 minutes over the course of his career. Dwight Howard is not especially good at avoiding fouls, but his career mark of 3.2 fouls per 36 minutes is a good margin better.
I expect Ibaka to become better at cutting down on mistakes as his career moves forward. The young big should get to, or at least near, where Dwight Howard is as a defensive force. The blocks will be there for Serge. The key in his progression will be refining the other aspects of defense.