When the rumors first started, about when the X100 was released (or maybe sooner), my biggest doubt was whether the hybrid viewfinder on the X100 could possibly be made to work with interchangeable lenses of various focal lengths, and particularly with a zoom. I still have my doubts, although the challenge of a zoom seems far more daunting than making it work with a few prime lenses. It looks like they're at least attempting to go in that direction. If they can pull if off, and the rest of the camera is as good as the X100 and X10 suggest it ought to be, this could be REALLY good!
At which point the challenge turns to making good lenses, and keeping them reasonably small, in an ILC with something like an APS sensor. Can they do it? Sony hasn't managed that trick yet (the 24mm Zeiss the case in point) but claims they're working on it. The key to the small profile of the X100, as I understand it, is that a lot of the lens is buried within the body. I guess that's more doable with a fixed lens than interchangeable lenses, but is this necessarily the case? Couldn't an interchangeable lens system be built with lenses that extend well back into the camera body? I don't know the answer - I'm assuming if it was easy, it would have been done by now. But that's the challenge in terms of my potential interest. I like a lot about Nex, but the m43 lenses (both quality and size) compared to the Nex lenses chased me back to the m43 fold for my 'system' camera. If Fuji can figure that one out and do a good kit of three or four great prime lenses, I could easily see selling a good chunk of m43 gear and going in that direction (although I'd probably keep my epl3 with the 14 and 100-300 for a couple of specific applications).
Who knows about sensor size... Fuji was claiming full frame image quality, but that could easily be a much smaller sensor packed with pixels and perhaps building their EXR technology into all shooting modes??? I don't have a clue, but I'm looking forward to finding out.
-Ray