Some points I'd like to highlight:
1) YouTube's growth was faster than anything we've previously seen. At one point he shows a chart comparing it to MySpace (where YouTube's growth rapidly surpasses MySpace) and points out that even MySpace during its time was faster than anything we've seen before it.
2) YouTube required the confluence of a few technologies that are fairly new, which is why YouTube didn't happen three years ago -- three years ago we didn't have all of: broadband in the home, cheap bandwidth, and cheap/pervasive recording technology like cell phones. He discusses Hot or Not at one point, saying that initially people were actually scanning photos of themselves for submissions. But digital cameras were becoming pervasive and Hot or Not quickly blew up at the same time. (You wouldn't believe it, but Hot or Not actually is a business. I talked to one of the founders at a party once and he said something like, "We at least make enough money to pay for a small full-time staff.") At the end of the talk he suggests that future killer apps will use similar combinations of emerging technology.
3) I couldn't tell from the talk how long they had been planning this (or whether he was just sketching in history in retrospect) but he summarizes some of the previous applications that indicated that the world was ready for something like YouTube. One example: he was initially skeptical of wikipedia but then he created an article and watched it grow, and from that learned that user-generated content can be of high quality. What I was surprised to see, though, was that of the five sites he discussed as major applications (wikipedia was one), the first was good ol' LiveJournal. (He even sorta half-way defends using LiveJournal here, saying that people are skeptical of this slide.) LiveJournal, he claimed, demonstrated (surprisingly at the time) that people were willing to share relatively personal things online with strangers, which is really quite similar to YouTube's format.
The most interesting bit to me though was that on that slide that describes LiveJournal he has a bullet that describes Xanga's numbers, because Xanga is a better example in terms of numbers. And while I don't consider LJ a failure, it certainly hasn't succeeded in the same spectacular way that other sites have. There are a number of interesting plausible reasons for this, I think. One is that it takes far more effort to maintain an identity via writing, especially personal writing, than it is to maintain an identity by just clicking on links (Friendster) or leaving nonsense comments (MySpace). And even those require more investment of personality and effort than looking at pretty pictures (Flickr) or funny videos (YouTube).
But here's another observation: YouTube had three cofounders. Two (or at least one, I assume two) were programmers and the third was a designer. LiveJournal obviously had Brad, and then there was also Anatoly or Whitaker or me as the other tech person, but there never has been a strong designer's influence (even now, I think?). And I don't mean design in the sense of pretty buttons -- I mean design in the important sense of making sure the site made sense to novice users or was easy to use. (For example, I think even now each new user generally has to have stuff like lj-cut tags, photo uploading, or even how their friends page works explained to them by other users. Yeah, it builds community, but it's still a serious barrier to new users.) I guess this is part of what Vox was trying to do, but to be honest I find it pretty confusing as well. I suspect I'm getting too old to use computers.