It would be nice to see a distinction between reinforced and wimpy doors.
Of course, that distinction is NORMALLY a matter of wood < metal < stone, and dwarves like using stone anyway. But maybe doors should be hard to break down based on the quality of the door.
"Reinforce door with iron" might be neat.
Can you honestly imagine a squad of goblins that wouldn't try sapping things, even if none of them had the 'sapper' profession? "Hey look--It's a door...And it's locked...I guess we'll just go away!" No, goblins would be prying at it with hands and spears, throwing rocks at it, throwing each OTHER at it, trying to stab it, trying to set fire to it. Even if it's made of stone. Goblins are fun like that. And eventually they'll probably get through, based on the quality of door at least. Five goblins versus a base-quality granite door? That door is going down. (Just imagine an unruly teenager with a crowbar and some determination.)
Hey look...It's a mason's bench! I guess we'll just leave everything sitting/hanging there instead of throwing these tools all over the place and tossing stones about. They don't have to -destroy- the components, just the building, leaving the components where they are. Hey look, it's a kitchen! ..etc. Goblins just couldn't resist. They should take lightweight loot too. Ooo, squads of thieves.
There's no point to only letting everyday goblins destroy doors, either. Otherwise you could block them with floodgates (which they oughtta be smart about destroying if there's something behind them--maybe you could tell?). Or even worse--just build some random jeweler's shop in front of a one-tile-wide gap in the corrodor so it's got an unwalkable tile in the way. Of COURSE goblins are going to bust it up!