Good sandbox adventures may take a bit of experience to create but presenting opportunity and a scenario as opposed to a scripted series of events is quite natural. It doesn't take thick book of material to show someone new to tabletop gaming how that is done.
The Moldvay basic DM section, in only a few pages, describes how to construct a basic scenario, pair it with a setting, and fill in the details. I read that with no prior DMing experience at all as an 11 year old and it seemed fairly straightforward and not unnatural at all.
My experience was much the same. My friends and I were all 12 or 13 years old when we started playing with the Mentzer edition Basic Set. We created our own adventures after only having seen the sample dungeon in that set (including wilderness adventures before getting the Expert Set), and it never occured to any of us to simply write a scripted series of events.
We had all read Choose Your Own Adventure and Endless Quest books, but none of us had any experience with computer RPGs (being young teens without computers at home in 1986).
It always seemed obvious to us that the game was intended to be played this way.
As for the interview with Mearls... he described pretty much exactly what I want out of a D&D rules set. At this stage of my life, I consider speed of play to be the most important aspect of D&D. There are many other things that I consider both necessary and important, but if we can't even complete a short adventure (say a small dungeon with a dozen rooms, half a dozen combat encounters, plus some roleplaying) in one four hour session, then something is wrong.
When you're married, own your house (which requires maintenance), work a full-time job, and have young children, it can be very difficult to maintain any sort of regular gaming activities. Thus, every minute of those irregular gaming sessions needs to count. In my gaming group, we share a mutual understanding that we all have to make some sacrifices in order to make these semi-regular gaming sessions happen. Thus, it is disrespectful to waste other players' time by constantly looking up rules, arguing about rules, or spending several minutes meticulously analyzing every possible option every time you take your turn in combat. Everybody needs to do their part to keep the game moving.
I never played or DMed 3.x regularly enough to really gain a comprehensive understanding of the rules. When I did wrap my head around the nuances of a particular rule, we'd end up not playing for six months and I'd forget it by the time we played again. And there were so many things that players could do which had a mechanical bonus or penalty, so naturally players would insist that I do it by the book rather than eyeballing it or making a quick judgement.
Give me a broad framework and some guidelines and let me handle the rest.