If you use Time Machine on OS X, you need to ensure your target drive is formatted with the correct Master Boot Record type.
Many upgrading Mac users have reported that Mail is “slow” in Snow Leopard 10.6. There are several things you can try to do to remedy the situation, but make sure you have a backup in place like Time Machine or are running an alternative like SuperDuper! or CarbonCopyCloner.
CocoaTech’s Path Finder tool is a versatile Finder replacement. One problem that you might have if you do any work with data, is importing CSV files that are in the UTF-8 format, and which contain multi-byte characters such as Japanese, into Excel.
Apple’s OS X Snow Leopard Server 10.6 implements Greylisting, an anti-spam technique based on forcing sending SMTP servers to “slow down” before they can deliver. This is great for reducing spam, but it also has the perhaps undesired effect of causing delivery delays. Sometimes really, really loooong delivery delays.