I have to admit, I hated this trend when it first came out, it reminded me of cheap college furniture at it's worst and '60's inspired interiors at it's best. Somehow, like many trends (and songs!) it wore me down and today I am coveting it.
I think the biggest obstacle to keep these from looking too casual. I think the best way to achieve this is to make sure these pillows are situated next to more formal seating options (chairs, couches, ect) and to pict the fabric they're made out of carefully. A fabric that seems too retro can make these pillows feel like too much of a throw back to the hippy error.
Below are my favorite options for floor pillows: