Didn't read the whole thread - in a bit of a hurry - but for what it's worth (about 2 cent, I'd guess), I think that in a perfect DF, invading enemies should be able to counter any strategy the player could possibly come up. If player builds moats, goblins bring bridges over them. If player builds walls, goblins bring trolls to tear down walls. If player builds lever-linked bridges and retracts them to drop goblins in chasm, goblins learn not to walk on player's bridges, or to only cross one person at a time. If player builds traps, goblins bring expert trapfinders. If player trains marksdwarves, goblins bring siege towers and heavy shields and stuff. If player consistently floods goblins with highly pressurized water, then.. well, I dunno, perhaps they learn to avoid that area and tunnel in somewhere else.
The point is that I feel players who want a real challenge should have no easy way out, not in moats or traps or siege weapons. Possibly in exceptionally clever fortress design, but even those should be possible for invaders to subvert, though it should be very difficult. Obviously all this is far from the current situation and will probably never be fully realized (even if Toady were to agree with me, which I don't know), but I think it would be a good goal to strive towards.
NB: I don't think every fort should be doomed to be invaded by eventually insurmountable forces, Moria-style. There should always be safe, cozy areas with no invading enemies where a reasonably diplomatic player could be at peace with all his neigbours and concentrate on building monuments and the like.