Do you think making the world an "open space" is a good idea?

Yes. But my wanting it so won't make it so.

Do you think that a defensive attitude makes attack more or less likely?

Leaving my front door unlocked is certainly not going to discourage burglars.

Do you think embassies have anything to do with diplomacy, and diplomacy has anything to do with dialogue? What message does a building expressing mistrust, suspicion, and paranoia project about a nation? How will that message, projected around the world, affect the ultimate fortunes of the nation sending it?

You're investing way too much in the symbolism of an embassy building. They have always tended towards the bland and bureaucratic (a decent enough reflection of the actual doings of an embassy). There are cultural centres and the like for the more sexy stuff.

And those open, glassy, friendly cultural centres don't get attacked because...? They aren't projecting their nation's values? The terrorists don't read art magazines? It looks panty-waisted to blow up a sculpture park?

The Dutch don't get attacked because they don't have global reach. Frankly they are too small to bother - for instance former colony Indonesia is now twenty times their size - and although they may still do awful things in the name of Shell Oil, they just aren't a lightening rod for world opinion like the United States. The United States is a target - now why it's a target is an entirely different story, but the different nations have different requirements for their embassies. Certainly you can see that?