It's not something that can be explained in words. If it was, then we'd all be amazing artists, we'd all be "able to draw like that." Nothing beats practice. However, I can give some tips.
Something few people seem to realize is the best way to detail well is to have a valve (or well made custom) map open in hammer/tf2 when you're making yours. Study everything that that map does to make those things look nice, constantly flip between yours and the valve/custom map and compare how they look. Observer where they place their windows and chimneys and vents and stains, how they use their textures, how they mix up the angles and rotations of their roofs, how they structure their architecture.
If you find an artist that says they don't use reference when doing their art, then you've found a liar. The advantage you gain from using reference is just too great to pass up.
For some specific tips, that style of building in badlands can look really good by simply just varying up the height, angle and rotation of the roofs. Even without anything else, just breaking up your big building into lots of "small ones," each with their own roof that's different from the surroundings goes a long way to making it interesting. Badlands goes the step further and makes each "small building" within the big building have slightly different wall placements, so parts are inset or protruding out from the average of the wall.