Below are some of my decorating pet peeves. They are the difference between a room looking like it was done on the cheap and one that looks well put together (but still done on a budget). I think I have blogged about this in the past but some of these are so common they need to be reiterated.
Curtains- Hidden tab (or grommet) versus tab curtains- all inexpensive but one definitely looks better then the other... and it helps to layer you window coverings.
Pillows- Lots versus no pillows- pillows on a sofa are a great (and inexpensive) way to bring in lots of colour into a room. My rule of thumb- the more the better.
Mat- Frames with mats versus frames without mats- mats are inexpensive and can be used to help unify several different sized (or shaped or coloured) frames.
Shower curtains- Fabric versus vinyl or plastic shower curtains- although you can find some unique looking vinyl shower curtains they should be used only to protect the outer fabric shower curtain.