I will copy here one of my posts, just for the records:
<<Yes, there's the happiness side effect, and about happiness i would like to spend some words:
i guess the happiness factor is "too easy to obtain".
From what i saw in my game, i just need to decorate a little the first level of my base, and, even if i dig to level 20, without a single decoration in alla the 19 floors, my happiness stays at maximum.
My suggestion here:
make the happiness factor based on the levels you discover, so that, ok, you want to dig deeper, but you need to decorate and create more living space, if you don't want your hapiness to drop!!
Also, make happiness low levels give much more malus.>>
Thinking about this, i guess that walls/floor sections (stone or wood), give too much happiness factor, i would try to experiment starting from 1/10 of the actual 1* happiness.
I think it's okay as it is now, because i only want to decorate some rooms and not have to build hundreds of statues and decorative plants by force just to get the happyness rating up. What i usually do is make my starting rooms and its surroundings pretty and then build a door where the "mine" starts. I don't want to beautify all the caves and tunnels i dig here, because well, it's a simple mineshaft in my eyes. Every few levels down i then pick a big cavern as an "outpost base" for my diggers (with a dinner table and some beds) and beautify that base too. This is usually just enough to boost happyness in the plus again. But it only works as long as walls and floors count full happyness points like they do now. Again, i think it's perfect the way it is. I really don't want to wallpaper everywhere i dig...