In response to a question asking what will be sent in the first missions to Mars, Musk responded that SpaceX's goal will be to transport colonists and make sure the basic necessities for survival were in place. He compared it to building the transcontinental railway. "A vast amount of industry will need to be built on Mars by many other companies and millions of people," Musk explained, which makes clear he's not planning on SpaceX colonizing Mars singlehandedly.
Musk addressed safety issues when it comes to using BFR for transportation around the Earth. SpaceX's goal is to reach (or even exceed) current levels of safety for passenger airlines. The focus of the Raptor engines is on reliability; they'll have a flak shield to protect them, as well as more engines than most airlines to ensure redundancy. "That will be especially important for point to point journeys on Earth," Musk said. "The advantage of getting somewhere in 30 mins by rocket instead of 15 hours by plane will be negatively affected if 'but also, you might die' is on the ticket.
Safety was also cited as the reason that the Raptor engines were downscaled from roughly 300 tons-force to 170 tons-force. After cheekily responding to a question addressing the downscaling with, "We chickened out," Musk cited the possibility of engine failure as the reason behind the decision. The ship's mass decreased between the previous IAC talk and this one; the engine downscaling is in proportion to that. "In order to be able to land the BF Ship with an engine failure at the worst possible moment, you have to have multiple engines," he explained "The difficulty of deep throttling an engine increases in a non-linear way, so 2:1 is fairly easy, but a deep 5:1 is very hard."
Musk also addressed his vision for a future city on Mars. The illustration provided shows ships that appear to have been used to construct the colony. One commenter asked whether that meant the first two spaceships would remain on Mars permanently as a part of the city. Musk's answer was succinct: "Wouldn't read too much into that illustration."
Another commenter asked about landing sites for the Mars base. Were the priorities on science or safety? As was a theme throughout this AMA, safety and survival are the paramount concerns. "Landing site needs to be low altitude to maximize aero braking, be close to ice for propellant production and not have giant boulders," he said. "Closer to the equator is better too for solar power production and not freezing your ass off."
If you're interested in technical specs and getting down into the nitty gritty, you should check out the full AMA for yourself. Musk answers quite a few detailed and well thought out questions (including a warning that the future design of the fuel tanker for the BFS will look "kinda weird.")