A gizillion articles have been published about Snow Leopard. I thought I would add my voice with some gotchas and goodies I came across.
Gotcha — Updating to Snow Leopard may change iCal Preferences to disable time zone support. To fix this, goto iCal > Preferences > Advanced .
Goodie — Mail in 10.6 has simplified creation of new mailboxes. Previously, many steps were required to create a new mailbox in a topic folder. Now it is a one-right-click (control-click) operation. Besides that, you can drag the new mailbox to where you want it to be.
Gotcha — In some cases after upgrading to 10.6 the Sync History may need to be reset. According to Mac OS X v10.5, v10.6: Resetting the SyncServices Folder, http://support.apple.com/kb/TS1627, Mail Rules may be affected.
"If you are syncing Mail Rules, in some cases Mail Rules will become deselected. Your Rules are still there, but you will need to re-enable any Rules you wish to use actively by selecting the checkbox next to them in the Rules pane of Mail Preferences."
Goodie — MacInTouch has a tip on showing invisible files in Open/Save dialogs. See http://www.macosxhints.com/article.php?story=20090915152215383