I'm basing many of my assumptions on the X100 and my time spent with that camera, too.
My guess is the RX1 will be much less quirky and a faster operating camera than the X100. While it doesn't have the optical / hybrid finder of the X series, that's about all that it is missing.
In exchange the RX1 will deliver, if our optimistic expectations are fully met:
- faster more reliable autofocus (it isn't happenstance that I list this attribute first)
- faster shot to shot time
- much improved EVF for those who want the EVF option
- probably less flare prone lens than the X100
- a real 35mm perspective with real 35 on 35 depth of field characteristics
- Zeiss rendering look. I ^still^ own a bunch of Zeiss glass, so +1 for this. Others may differ.
- state of the art Sony sensor, with increased resolution and real estate
- improved ability to crop-zoom out of the camera while keeping quality up
- if the D600 sensor is the same as A99/RX1 as is thought, fantastic dynamic range
- sensor spec offers enough future proofing to be servicable for years for most folks
- vastly better video capabilities, for those who care about video or will in the future
- lens/aperture ring ergonomics look better (I found ring on X100 awfully cramped)
- manual focus actually usable
- no X100 Aperture Dance (one can hope)
That was off the top of my head.
As mentioned up thread, I hope Sony takes a page from Fujifilm's book and delivers one or more lens converters to provide additional, optical, focal lengths from this "fixed" FL camera. That is an advantage for Fujifilm at present although the adapter is wider not longer as I'd want.
I greatly enjoyed using the X100 until it started to fail. Still, the camera and its firmware had enough flaws that it got in my way more than I really want my primary every day camera to get away with, so I sold it... but I still do miss the simplicity of one camera one lens in a small, very high IQ producing, package.
If the output and construction quality of the RX1 is all that is expected, then I've no doubts it'll meet my needs for the 80% plus shooting I do with a 35mm or equivalent field of view camera. Yes, the RX1 is expensive, but so was the X100 at $1,399 or whatever I paid for it. I figured at least $1,000 premium for a full frame version of the X100 so the $2,800 number isn't too far distant from that wild assed guess.
Speaking of guesses, maybe some of them are even 'educated' guesses, but it's all supposition at this point.
Can't wait for the pudding to arrive for proof.