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If it is translucent mirror it could mean very high compatibility with legacy/adapted glass.
I'm more interested in this statement... why using a translucent mirror means higher compatibility with legacy lens? Canon 5D is quite hard to beat for legacy lens compatibility (SLR lens only of course).
As a NEX-7 user, I don't think the current EVF is better than a fullframe OVF. It has to much bigger with much higher dynamic range to beat a good OVF.
A900 was much cheaper than D3X, that's why it was tempting to many. Now D800 is actually quite affordable for those who really NEED that quality. A99 with a fullframe sensor alone can't attract new customers. Keep in mind that the target group are most likely has already invested in Canon or Nikon.
I use Nikon DSLR, for me, that most appealing thing about Sony Alpha is body stabilization and its lens has full compatibility with NEX (via their adapter). IF A99 can use more lens (Nikon, FD, Leica R) and provide stabilization, it would be very interesting.
IF 24MP means better DR and high ISO than 36MP, I would take 24 over 36.
oh, I would certainly buy a full frame NEX7 like box if they make one.
The A99 is rumored to have an improved EVF, and, although there are certainly advantages to a good OVF, it's hard to argue against the advantages of an EVF when it comes to manual focusing adapted lenses, because of image magnification and peaking.