HeidiL would be a good one to explain it too, but I think (from what I've seen & understood) that Wyvern's on the mark. The "breeze block" bricks were relatively weighty themselves, and when we made solid structures it took 2 people to lift one chair, so imagine the weight of 2 floors of solid bricks...
Well downwards compression strength was never an issue, and beams joining the tops of the walls would have dealt with a lot of the wobbliness of the walls, which were basically free-standing structures. The crack around the door was caused by the fact that the base wasn't completely flat, something which was exacerbated by the ply warping when it rained. I believe the height discrepancy on the two sides of the door was already there when the lintel was put in place, which would have meant that there was a lifting stress on the piece as soon as it went in. If it had happened when it already had a load of bricks on top of it, then it might well not have managed to lift it - at least not immediately.
Then there was the fact that the wooden beams inside moved around in the wind before the walls were all the way up, and so bashed into the insides of the walls, which caused more shifting and a few more cracks. We'll gloss over Vic chucking things down from the roof which bounced and smashed holes in the bottom of the walls... though that would have been the main argument for thickening the sides of the bricks - preventing people from kicking holes in the walls. However, they would have been a lot fiddlier to construct, which would have made the project far longer - enough people were building malformed bricks as it was, never mind the additional time to build something more complicated - and it would also have been much more difficult for us to dismantle it once we were done.
That said, I do think that given a completely flat, secure base, time to develop better beams - something I wish I had the money to throw at experimenting with - the right constructors and a different H&S-calming support structure (I also have ideas about that, partly based on the suggestions of a brickhead who turned up, spent a day criticizing everything and not doing much building) that a structurally-sound two-storey lego house is entirely possible. Even with a roof. Wind and waterproofing would require some more traditional materials, but those shouldn't be too hard to incorporate...
One thing that I think needs to be said though, is that if any of us deserved to be at that house-warming, it was John, Julia and Brandon, who built some really amazing things, most of which got little, if any, screen time.
Oh and the cracking of the divan when James sat on it? It didn't do that with a number of us sat on it eating pizza... (do not get me started on trying to explain to the pizza delivery boy where he was going) but that might have weakened it a bit.