Some changes seem to be fairly standard, adjusting weapon speed and damage and the like. Others seem to fix more fundamental issues, such as making sure players can't join private parties without first being invited, or ensuring that the team voice chat channel can't be heard by the opposing team.
A handful of single-player issues have been rectified as well, though it's difficult to say how significant the issues were without a frame of reference. Among the changes are making the default difficulty normal instead of easy, fixing certain instances of unresponsive controls, improving single-player stability and fixing "progression stoppers."
The Warfighter blog notes that the patch is designed to "ensure that your experience when the game ships is outstanding." However significant the changes are, it's apparent that EA found them important enough to hold back press review copies until the patch was deployed. Joystiq's review will be up as soon as possible.
You can read the full patch notes here.