A problem I have with this movie and, with certain episodes, of Star Trek is this -
I am living in the present. I know that our government has taken preventive steps to protecting itself from attack. I know that they can't protect everything - there are breaches. However, these breaches are rare, and the government does take partial to full steps to prevent these occurrences from happening again. I am assuming, as a student of history, that a government in the twenty-fourth century will be far more advanced than us in the identifying and quarantining of threats. So, when I see an attack on a government building, I am in a state of disbelief. I know that this wouldn't happen, especially after an attack has already happen. The conference would be held in a secured facility - out of mind, out of sight of the public - and that Khan wouldn't be simply able to fly a gunship to the conference room and kill people. When I see an event in a movie, and it makes it appear that the people are less advanced than us, and this is allegedly the future, I feel the film has lost some of its credibility with me.
I believe that if you are to write a film set in the future, show that the people have actually precedent behind them and that they are working from centuries of acquired knowledge.
If you are going to show Khan killing people, then do it intelligently. First off, people like Khan wouldn't get themselves dirty. They would hold themselves above the fray, and have others do the work for him. (The father destroying the complex has some verisimilitude.) Leaders who get dirty betray weakness. Secondly, show the process by which Khan's minions overcome Starfleet's defenses and are able to attack the conference. Thirdly, make the attack be less random, and more focused. Fourthly, illustrate how Starfleet has learned who was behind the attack. If you are involving an intelligence group, use that to some advantage. Have them either involved with the attack, or have them be the victims of the attack.