We're talking about hitting an object half the size of Manhattan with something the size of a steamer trunk from 83 million miles away. If our weapons are as accurate as our space probes, we can hit hit a fly with a BB around the world a week from now. Asking why the fly was shot through the wing instead of the head seems to be expecting a bit much.
Also, I never would have suspected that there would be so much cratering on a comet. We keep hearing that they are "dirty snowballs" but this 0looked just like one of the moons of Mars or an asteroid.
Yeah, that struck me, too. The crater was deeper and the debris cloud larger than any of NASA's animations predicted. All of the best guesses at comets' compositions were pretty vague -- so I don't know whether there was more dust or more vapor than expected. That's pretty much what this mission was supposed to find out.
The flyby and impactor were each designed to navigate autonomously in the last part of the mission, when they couldn't be flown from Earth because of more than seven minutes' lag in communications. If a spacecraft can maneuver itself into the path of a fast-moving object without guidance from the ground, that could have interesting implications for ballistic missile defense.
If the impactor had hit head on the body of the comet would have masked the impact from the flyby probe. Course corrections were made in the last few minutes to ensure the flyby probe had the best possible view.
Also looked like there was a frozen water feature there too. A lake or a pond.