I've really enjoyed reading your series of columns, I think they're very informative for people just starting off trying to get into pen and paper type games, whether as players or game masters. Seems like a very solid basis to try it out and see if you like it and such, I just can't help but feel that it doesn't really teach some of the more important pieces to game masters and roleplayers alike.
For example, when I first started playing rpgs I had no idea how to really 'make' a character. As in, how my character would act, what his motivations and goals might be, or even why he would be in the setting in the first place! I felt like I sort of had to go in blind and throw my character into the fray (of roleplaying, not combat) and see what happened. Often I found that what I envisioned was either completely lackluster and boring or just didn't work period, and needed to be adapted or tweaked so that I could actually have fun playing the game. I mention player difficulties because I feel like one of the ways you become a really good game master is to be a player first and really understand how to create interesting characters, otherwise how else are you going to engage your players?
And then I got wrangled into being the Dungeon Master for a D&D campaign. I think your advice is pretty spot on in some places in that a top down design is the easiest way to provide a framework to use for your players, but I found that with experienced players (and roleplayers) who knew how to work together and use their skills properly, and enjoy doing unpredictable things and creating oddball solutions to difficult problems, no amount of preparation on my part could provide what I would need to be able to deal with the sorts of things they would do. I ended up having to completely improvise inside the framework of the world I had created, (again, top down is great, without that framework I would have been screwed) and let me tell you, for someone who has never been a dungeon master before, that can be scary. After our five hour sessions, I would feel more exhausted than if I had worked all day, my brain would be that fried. I guess what I'm getting at is that when you don't want to or even can't railroad your players in any way, it becomes extremely difficult to adapt on the fly. Experience is probably the best medicine for this, but it would be interesting to hear your insight into dealing with unexpected situations.
Again, loving the series, keep it up!