Overzealous Decorative Painting.
Decorative painting can be a beautiful addition but you have to know when to stop! Four feature walls in a small space is not OK. Mary Douglas Drysdale says "brilliant painting is elegant and polished, but go too far and you end up with vulgarity".
Bright Overhead Lighting
There's nothing worse than feeling like you're on stage shrouded in spotlight! Barclay Butera recommends using dimmers and soft white bulbs, "lighting shouldn't wash down on you, it's harsh and unflattering".
Single Source Lighting
And while we're on the topic of lighting, a single light source is a no-go-zone as well. The key to good lighting is layering it at different heights. Alex Tanksley says, "don't rely on only one kind of light source. Mix a variety of overhead and floor or table lamps".
Furniture Pushed Up Against Walls
Don't set furniture up as though you're waiting for a main act in the centre. Furniture placed around the wall creates a seating arrangement that makes conversation difficult. Whitney Stewart believes that the guiding principle should be to "create intimacy, how can you have a conversation when you have to shout across the room?" When setting up a room she imagines the sofa talking to the chairs and the chairs talking to each other. I like that.
Lamps that are wildly out of proportion on either ends of a sofa drive David Kleinberg crazy. "Don't get too creative with lamps, keep it simple".
Don't over fuss with your living room pillow arrangement. Steven Sclaroff can't live with karate-chopped pillows, "that look would never occur in pillow nature". Pillows have a nature now? No, but I agree, don't like the karate-chop look either (though would never have thought to describe it so perfectly).
Hope you're all having a nice day :)