The advantage? All the apps are now much smaller (I suspect the 32 bit apps in the past had to still honor 64 bit boundaries but utilizing only half the instruction and data space). Smaller means faster load and execution times, less swapping etc. So even if there were no new features, this alone was a great update.
Faster execution of all the system apps. Definitely feels more snappy.
Less diskspace. I got back around 6-8GB on my two Mac systems at home.
None really except it will ask you for permission for apps you installed because it thinks you're using them for the first time.
Make sure you backup your current system first. I do a full backup to a dmg file and time machine backup. Good job I did.
The upgrade re-installed apache and deleted my conf files. The lesson learnt is if you think you have custom files in directories outside your user directory, make sure you back them up, you'll have to copy them back after the install.
MySql lost its symbolic link and didn't start automatically.
My iTunes crashed trying to access my iPhone but resynced successfully after relaunching. Some anxious moments here!
Xcode didn't think my iPhone was a valid development device until I rebooted my iPhone. Tip: shutdown iPhone and hold down the home button while restarting the iPhone until you see the Apple logo.
Takes about 45 minutes to do the upgrade. For developers, XCode is supplied with the DVD. Nice touch.
So for the very first time in my life, I'm glad it snowed in August!