Ability to dock both panels on the same side so you can reduce mouse movement during work.
"I'm always moving the mouse from the far right side (structure panel) to the far left side (settings panel). The number of miles I cover in a day must be staggering."
- Eric Embacher (via facebook: Official Oxygen User Group)
Why right side is natural for lot of people?
"People work in tools like Adobe Illustrator, Adobe Photoshop which have this kind of layout and this comes natural position for design people"
Why not floating panels inside idea?
This could need to make lot of under hood work to take full advantage of floating so you can drag to other screeen. It would require solving CORS problem, communication between two windows etc. and it could be fragile.