Like many of the people I visit regularly, I sprung for Tiger simply because I'm hooked on Macs. Though I can still buy at the educator's discount because I'm a retired teacher, I think Tiger might have been worth the full amount this time.
So far, my favorite addition has been the ability of Safari to automatically discover RSS feeds for sites. It's discovered at least ten or fifteen feeds that I couldn't discover before; not only does it discover RSS feeds but it automatically adds them to NetNewsWire. I love it.
Another seldom-mentioned feature I love is the addition of the Oxford Dictionary at a system level, meaning that you can automatically look up any word you discover on a web site by selecting the text and clicking while holding down the control key. If the dictionary can't find the word, you also have the ability to search Google using the same key commands.
While I'm not as impressed with Spotlight as some are, it has proven helpful in some of the searches I've conducted in trying to explore all the possibilities of Tiger.
Finally, "smart" mailboxes have proven to be real time saver while using Mail. I've only set up a few new mailboxes, but they have proven helpful in finding what I want to find in the shortest amount of time possible.