He got snubbed no doubt, he should have been in there. Good motivation to keep improving though, hopefully he reacts to this the way you'd like your All-Star quality player to react to a snub like this.
Brook's averaging slightly more blocks per game than Smith and more than double Horford's single block per game average. And he does it in only 30 minutes per game, which is about 6 or 7 fewer minutes per than either of them.
Duncan in particular has always been a good shot blocker, but I'm not equating blocks to good defense. In Brook's case, his increased blocks per game average is a result of his improved defense. Ibaka is actually the opposite - his bpg went down because his overall defense got much better this year.