The X100 viewfinder was one of the reasons I didn't like it. So-so optical finder, easily too cluttered, and poor EVF.
The new M differs from the M9 viewfinder in that the frame line illuminatir is now an LED light source instead of ambient light, just like the M9 Titanium. This means the frame lines can always be consistently bright and adjust in brightness for ambient conditions, and their color can be set by the user between red and white.
The frame line mechanism is the same as in the M9 other than the removal of the selector lever for previewing. I don't use it often enough to bemoan its loss personally, and removing it helped keep the price in check, I'm sure. Perhaps it will reappear on a future "P" version.