For example, the BOM file indicates that changes were made to iChat's balloons, boxes, compact, and text view styles. But comparing those views on my 10.5.6 and 10.5.7 machines reveals no visible differences. I could, of course, use a program like diff (or FileMerge, part of Xcode) to compare the files--text files, at any rate--to see where the differences lie. But with 449MB worth of data, that would be a horrendously time-consuming project.
Instead, I launched each of the modified programs, looking for any visible differences in menus or preferences, as compared to the 10.5.6 version of the same program (running on a not-yet-updated machine). However, nothing obvious jumped out at me in the way of new features in any of these programs. Then again, I've only been able to spend a few minutes comparing each program, so perhaps I've overlooked something--if you spot any new features, feel free to post them in the comments.
In System Preferences, there are changes to the following panels: Bluetooth, Desktop & Screen Saver, Energy Saver, MobileMe, Parental Controls, Print & Fax, Sound, Startup Disk, Time Machine, Trackpad, and Universal Access. As with the applications, however, I couldn't spot any visible differences when comparing the same panes on 10.5.6 and 10.5.7 machines.
Beyond the changes to applications and System Preferences, there are changes in low-lying technologies, too. The BOM file reveals changes to the iLife Media Browser; EPSON scanner drivers; Flash and Shockwave players; Quick Look generators for iWork; 802.11 and RAID frameworks; Spotlight importers for Office, iWork and (somewhat of a surprise) AppleWorks; updates to many Automator actions; a slew of changes to Bluetooth, including changes to the Bluetooth Setup Assistant; and lots of changes to audio and video (both ATI and Nvidia) drivers.
Finally, there were a ton of updates on the Unix side of OS X. Most notably, Apache 2 (2.2.11) and PHP (5.2.8) were both updated, as were some Perl libraries, various Python and Ruby frameworks, and a whole bunch of Unix utilities. There are also updates to many of the X11 Unix programs.