If you haven’t, and you want to play in it, ***SPOILER ALERT***
First off, he thinks you need to blow stuff up. If there’s a favorite tavern the PCs enjoy hanging out in, or a particular lord they love (or love to hate) or whatever, he suggests you have the Skyfortress giants come down on it like a ton of bricks. And he’s not talking about a little raid that knocks over tables and kidnaps a few peasants. He suggests you ask yourself:
If the Giants attack by surprise at night, as is their wont, is there any possibility of survivors or would visitors find nothing but a collection of bloody smears?
Death Frost Doom is (in)famous for the possibility that the PCs might unleash a plague of zombies on the world. Broodmother Skyfortress assumes something nearly that bad is happening already.
I love this idea when used in moderation (as Jeff himself suggests in the book). A destructible world is more real, more immediate to the players, and far more interesting. Being willing to blow up the Keep on the Borderlands (illustrated beautifully on the back cover of the book) tells players that they’re gaming in a truly dynamic world where their actions (or lack thereof) will have real consequences. That’s cool.
On the other hand, if the players never know whether or not all elves will be transformed into cat-people, or all wizard spells now result in explosive flatulence, or all the gods are simply going to vanish at the drop of a hat, it actively prevents them from investing in the setting. Doing one of those things over the course of an entire campaign might be cool. Doing one (or more) of those every time you sit down to play tells the players that nothing is trustworthy, and they’ll just avoid investing in any of it.
While that’s fascinating all by itself, Jeff’s after more than just blowing up your campaign. He wants to blow up your entire game.
When fighting the giants, Jeff suggests you just not bother having the players roll to hit. Yep, they always automatically hit the Giants. However, they have obscene numbers of hit-points and the first five points of damage from any attack don’t count. Attacking them with mundane weapons (or even lesser enchanted ones) is barely going to scratch these monsters. Normal combat is going to lead to TPKs; the players simply can’t put enough hurt on these monsters fast enough to take them down (unless they’ve got access to some really nasty magic). And, by eschewing the sacred d20 roll, players should immediately understand that this is an unusual situation.
Jeff calls this Mechanical Alienation, and it’s a cool technique. Again, it’s probably most potent when used rarely (I’m thinking of invoking it at the end of one of my current campaigns), but it does drag into the middle of the table the central question of what your RPG is. Gygax is famously credited with saying, “The secret we should never let the gamemasters know is that they don't need any rules.” This is that idea turned up to 11. It’s not quite free-form RPGing, but rather understanding that all the rules are there to promote the fun. Rules that don’t promote the fun should be tossed aside, maybe for good, maybe for only a few moments. It would be too much fuss and bother for us DMs to create a completely original set of rules for each encounter (and if you did, you’d destroy player trust in your campaign; see above). But there is a time and a place to set the book aside and recreate the game in a more fun shape. At its heart, this is what Broodmother Skyfortress is all about.