gg 1.2 mainly represents improvments in usability and compatibility.  Numerous changes were made to make it more friendly and compatible.

compatibility/install improvments:
-GoodGallery's self diagnostics are now harder to break.  in 1.0, certain PHP configurations would either disable error output or 
just display "500 internal service error"; but in 1.2, error reporting is forced on which allows the client side code to diagnose the problem.

-Memory limits are now modified by the script if the php configuration prevents proper operation

-Permissions errors are detected and alert the user during initial build

-XHR build requests are limited and the user can set the max number of simultanous XHR's.  This prevents the script from running wild on cpu usage
in a shared hosting environment.  The default is "safe" (only 1 at the same time)

-No longer displays the page number if there is only one page [of thumbnails]

things that ALMOST made it into 1.2:
-built-in uploading ability like http://lagbag.com/bucket for admins

-intelligent browser cache control to avoid firefox "cached failures" and to reliably force reloads when firefox failsto load a file that exists
(in short, packetloss compensation)