The sensor, the sensor, and the sensor. The RX1 has bitten me with a bad case of the full frame bug. I'd like some sort of full frame ILC, but I'm still thoroughly undecided if I'm willing to shoot one with the smaller lenses I'd be willing to use. But I like the sensors a LOT.
For longer or ultra-wide or zoom lenses, where full frame lenses start to get really huge, I fully expect and intend to stay with m43. Whether there is a set of small prime lenses I like enough to shoot with the Df is a question I'm grappling with and having my doubts about. But modern full frame sensors are a real treat that I'd like to have available with more than one lens if the other compromises aren't too great.
-Ray