What are the changes/limitations of the 64 bit Photoshop plugin?

There are several important changes:

  • The 64 bit plugin can't use the Adobe CMM (which allows for Black Point Compensation), since Adobe hasn't yet released one for 64 bit Photoshop
  • The processing method for monochrome photograph printing has been changed to improve the representation of dark areas. Northlight Images found the main change was an opening up of the shadows except for the 95-100 range where they've been compressed. This change means that a new linearizing profile is needed for those using the Quadtone Rip Linearizing to get good screen to print matching.
  • You cannot apply Favorites, History, or Custom Paper Size created with V5.00 or later with Plug-In earlier than V5.00
  • The X000 printers are not currently supported

You can read more details from Northlight Images Review and User Notes.