We have a big loft right when you go upstairs where the bedrooms are. That is our playroom. We keep it neat, because we walk through it and it's the first thing you see when you get upstairs.
The key is to have toys organized. You can get cute baskets and storage bins/shelves.
This is what we have:
http://www.ikea.com/us/en/catalog/ca.../series/19027/
We have the light wood storage systems with blue and green storage bins.
When my kids want to bring toys downstairs to the living room (main floor) they grab a bin and bring it down. When they are done, the bin goes back upstairs to the loft. We try to limit the toys on the main floor, and this helps.
I have friends who have the white or light wood storage systems on their main floor in a converted office or formal dining room. I think it would be fine!